API error handler

[godotalgorithm]

This an an error handler for the diskless/stateless API built on top of MOPAC's existing error handler. Errors trapped during a calculation are collected and reported back to the user in the "properties" output. Additional checks are made on the validity of the API inputs.

I've also added more tests to cover all of the basic features of the API and the various bug fixes that were needed to pass all of these tests. Most of these bugs were in the API itself, but I've also repaired a few (but not all) of the problems related to small-unit-cell calculations [#212]. The testing coverage still does not cover all possible combinations of features, but it's broad enough now to cover what I expect to be the typical usage of the API.

Status

• Ready for merge

[codecov]

Codecov Report

Attention: Patch coverage is 93.26221% with 120 lines in your changes missing coverage. Please review.

Project coverage is 67.94%. Comparing base (b0b0852) to head (83c2634).

Files with missing lines	Patch %	Lines
src/interface/mopac_api_saveload.F90	51.78%	54 Missing ⚠️
src/interface/mopac_api_finalize.F90	59.25%	33 Missing ⚠️
tests/mopac_api_test.F90	98.98%	15 Missing ⚠️
src/interface/mopac_api_initialize.F90	75.60%	10 Missing ⚠️
src/mopend.F90	20.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #213      +/-   ##
==========================================
+ Coverage   67.23%   67.94%   +0.70%     
==========================================
  Files         336      336              
  Lines       75427    77013    +1586     
==========================================
+ Hits        50713    52325    +1612     
+ Misses      24714    24688      -26     

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