grav_undef() to gravitationally unbend observed light

[attipaci]

The inverse function for grav_def().

Also:

• Fix up test cases but initializing all data to avoid unreproducible behavior in tests.
• Use precompiler constants to define max number of iterations for inverse methods.

[codecov]

Codecov Report

Attention: Patch coverage is 76.19048% with 5 lines in your changes missing coverage. Please review.

Project coverage is 98.65%. Comparing base (7e6f810) to head (1dfa376).

❗ Current head 1dfa376 differs from pull request most recent head 199af4c

Please upload reports for the commit 199af4c to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
- Coverage   98.78%   98.65%   -0.14%     
==========================================
  Files           3        3              
  Lines        1983     2004      +21     
  Branches      370      378       +8     
==========================================
+ Hits         1959     1977      +18     
- Misses          7        9       +2     
- Partials       17       18       +1     

Flag	Coverage Δ
unittests	98.65% <76.19%> (-0.14%)	⬇️

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Files	Coverage Δ
src/novas.c	98.44% <76.19%> (-0.16%)	⬇️

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