Probability Distribution and Statistical Functions -- Gamma Distribution Module

[Jim-215-Fisher]

This is the second round of probability distribution and statistical functions, continuing from #240. Since the whole modular structure has been changed, each distribution will have its own PR.
This PR contains gamma distributions. Files uploaded are:

• src/stdlib_stats_distribtuion_gamma.fypp
• src/CMakeLists.txt
• src/tests/test_distribution_gamma.f90
• src/tests/CMakeLists.txt
• doc/specs/stdlib_stats_distribution_gamma.md
• doc/specs/index.md

Link to the open-access article on which this PR is based.
P.369 (7/10) has a summary of the final algorithm.

[Jim-215-Fisher]

This one is ready for review.

[Jim-215-Fisher]

Compilation failed due to Build-API-Docs failure of other modules. Error message is AttributeError: 'FortranSubmoduleProcedure' object has no attribute 'module'. What does that mean?

[14NGiestas]

@Jim-215-Fisher it looks related to the ford issue ford#466.
Apart from that, this PR is LGTM. The only thing I'm unsure about it is: How was your approach to testing, and where did you get the expected output?

[Jim-215-Fisher]

Thank you for the info. Regarding the testing, gamma random number is tested with common Pearson's chi-squared test, pdf and cdf were tested against results from scipy or excel formula with the same parameters.

@Jim-215-Fisher it looks related to the ford issue ford#466. Apart from that, this PR is LGTM. The only thing I'm unsure about it is: How was your approach to testing, and where did you get the expected output?