adds compatibility flag (for older version comparisons); updates adjacency array setup

[danielpeter]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 80.83333% with 23 lines in your changes missing coverage. Please review.

Project coverage is 37.20%. Comparing base (ee26690) to head (5a25fa1).
Report is 4 commits behind head on devel.

Files with missing lines	Patch %	Lines
src/specfem3D/setup_sources_receivers.f90	87.09%	12 Missing ⚠️
src/meshfem3D/moho_stretching.f90	16.66%	5 Missing ⚠️
src/shared/rthetaphi_xyz.f90	20.00%	4 Missing ⚠️
src/meshfem3D/compute_element_properties.f90	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            devel     #848      +/-   ##
==========================================
- Coverage   37.31%   37.20%   -0.12%     
==========================================
  Files         279      279              
  Lines       64562    64557       -5     
==========================================
- Hits        24091    24017      -74     
- Misses      40471    40540      +69     

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