[femutils] Minor refactoring of classes BSRFormat and BSRMatrix

[grospelliergilles]

Make big methods of BSRFormat and BSRMatrix non inline.

[codecov]

Codecov Report

❌ Patch coverage is 91.79104% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.01%. Comparing base (50a0cec) to head (3d0252b).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
femutils/BSRFormat.cc	90.54%	23 Missing and 8 partials ⚠️
femutils/BSRFormat.h	97.29%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #310      +/-   ##
==========================================
+ Coverage   69.00%   69.01%   +0.01%     
==========================================
  Files         112      113       +1     
  Lines       14265    14276      +11     
  Branches     1944     1953       +9     
==========================================
+ Hits         9843     9853      +10     
- Misses       3959     3977      +18     
+ Partials      463      446      -17     

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