Draft: Work on interfaces in tracking core.

Builds upon !1882 (closed) and discussions therein.

Changed description after discussions. The purpose of this MR is now a general restructuring of interfaces.

The classes ca::TrackFinder and ca::Framework are not clearly separated by any interface and depend on each other to such a strong degree, that having them be separate classes creates more problems than it solves. With this MR I propose merging them into a single class, to simplify things.

This work is currenty still ongoing, hence the draft flag. While working on this, I encountered one issue, that I wanted to discuss. I noticed something odd, when running the online binary, using the non-modified code from the current master branch, multiple times using the following command:

./cbmreco -i ~/lustre_tsa_data/3012_node9_06_0017.tsa -r 3012 -o 3012.tracks.rra -O Track --omp 20 -p ~/cbmroot/parameters/online/ --steps Unpack LocalReco Tracking DigiTrigger -n 1

What I observed is that the number of produced tracks fluctuates slightly between executions of the binary. Here are two excerpts from the command-line output, from subsequent runs:

----- Counters:
Key                            Total                    per full routine calls   per sub-timeslices       per reco tracks          
full routine calls                                    1                        1              3.71292e-06              5.37195e-06 
sub-timeslices                                   269330                   269330                        1                  1.44683 
reco tracks                                      186152                   186152                 0.691167                        1 
reco hits                                       3892877              3.89288e+06                  14.4539                  20.9124 
used reco hits                                   707570                   707570                  2.62715                  3.80103 
triplets                                        9350788              9.35079e+06                  34.7187                   50.232 
MVD hits in tracks                                    0                        0                        0                        0 
STS hits in tracks                               295449                   295449                  1.09698                  1.58714 
MUCH hits in tracks                                   0                        0                        0                        0 
TRD hits in tracks                               249258                   249258                 0.925474                    1.339 
TOF hits in tracks                               162863                   162863                 0.604697                 0.874893 

----- Counters:
Key                            Total                    per full routine calls   per sub-timeslices       per reco tracks          
full routine calls                                    1                        1              3.71292e-06              5.37372e-06 
sub-timeslices                                   269330                   269330                        1                   1.4473 
reco tracks                                      186091                   186091                  0.69094                        1 
reco hits                                       3892878              3.89288e+06                  14.4539                  20.9192 
used reco hits                                   707398                   707398                  2.62651                  3.80136 
triplets                                        9346721              9.34672e+06                  34.7036                  50.2266 
MVD hits in tracks                                    0                        0                        0                        0 
STS hits in tracks                               295338                   295338                  1.09657                  1.58706 
MUCH hits in tracks                                   0                        0                        0                        0 
TRD hits in tracks                               249274                   249274                 0.925534                  1.33953 
TOF hits in tracks                               162786                   162786                 0.604411                 0.874766 

@se.gorbunov @s.zharko Can you comment on this? I haven't observed this when running in single-threaded mode, so it might be some kind of race condition.