L1: code clean-up and account for nullptr field case

• dFEI keys are replaced with internal link keys
• nullptr field case is taken into account

added Reconstruction Tracking labels

requested review from @v.akishina

assigned to @se.gorbunov

added 5 commits

• cfce7b7e...bbdb5df8 - 2 commits from branch computing:master
• 1690917f - L1: Duplicating code for STS hits matching was removed
• 6df049ed - L1: dFEI keys were replaced with CbmL1LinkKey struct
• 0c236493 - L1: case of nullptr field included to CbmL1

Compare with previous version

Dear @se.gorbunov, @v.akishina,

you have been identified as code owner of at least one file which was changed with this merge request.

Please check the changes and approve them or request changes.

added CodeOwners label

@f.uhlig, as far as I understand, you don't like using MC information in CbmL1 in general?

We need it in general for debugging and currently for performance evaluation.

The way the MC information is used will be reorganized, and this MR is the first step towards this reorganization. Sergey just cleaned up a bit the mess we have with the indices.

resolved all threads

merged