Partially complete C interface

[mathiasbourgoin]

This is linked to #5.
It adds some functions to the C interface.

[codecov]

Codecov Report

Merging #6 into master will decrease coverage by 0.28%.
The diff coverage is 35.29%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
- Coverage   20.16%   19.87%   -0.29%     
==========================================
  Files          20       20              
  Lines        2242     2274      +32     
  Branches      473      473              
==========================================
  Hits          452      452              
- Misses       1348     1380      +32     
  Partials      442      442              

Impacted Files	Coverage Δ
src/mammut.cpp	23.63% <35.29%> (-9.70%)	⬇️

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[DanieleDeSensi]

Thanks Mathias ;)

Best,
Daniele