added C-interface c_gcp_call

[jkn93]

I've added a C/C++ wrapper to handle Fortrans weird 4-bit-booleans and strings.

Can be used with the following declaration (C++):

extern "C" void c_gcp_call(int* n, double* xyz, double* lat, int* iz, double* gcp_e, double* gcp_g, double* gcp_glat,
bool* dograd, bool* dohess, bool* pbc, const char* method, bool* echo, bool* parfile);

Cheers...

[awvwgk]

Can you add a C header with the signature as well?

[jkn93]

added header file as well as one test (tests/c_api). I'm not sure how to integrate that since I usually don't use meson...

[jkn93]

Regarding the test-file:
I used the test-system of the R2SCAN-3c SI, but I didn't check the HF3c value.
Feel free to extend the tests...

[TyBalduf]

@awvwgk would you be able to retrigger the CI checks? I was going to see if I could help clean up this pull request, but the errors from the last CI run have expired.

I was looking into using this library for a project and a C-api would be useful.