TridiagSolver (dist): STEP2 Make rank1 work just on local non-deflated eigenvectors

[albestro]

• stablePartitionIndex now computes k_lc together with i6 and i2 needed by rank1
• rank1 solver is single-threaded (next PR will make it multithreaded)

TODO

• [x] snake_case
• [x] fix doc
• [x] check TODO

[albestro]

cscs-ci run

[codecov-commenter]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (b99bb16) 94.05% compared to head (84efe67) 93.99%. Report is 1 commits behind head on master.

Files	Patch %	Lines
include/dlaf/eigensolver/tridiag_solver/merge.h	95.97%	2 Missing and 5 partials :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #996 +/- ## ========================================== - Coverage 94.05% 93.99% -0.06% ========================================== Files 145 145 Lines 9068 9032 -36 Branches 1160 1159 -1 ========================================== - Hits 8529 8490 -39 + Misses 321 320 -1 - Partials 218 222 +4 ```

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