Addition of the Mathieu even and odd functions

[mahdi-sasar]

Hi everyone.
The old repository here only allowed for the calculation of the characteristic values of the Mathieu functions. I decided to add the actual definitions of the Mathieu sine and cosine functions using the Fourier series expansion of these functions that is available in many textbooks.
Hope this is useful and can aid others as well. I personally use them extensively now for some quantum mechanical studies where using Mathieu functions as basis sets is computationally beneficial.

[codecov]

Codecov Report

Attention: 76 lines in your changes are missing coverage. Please review.

Comparison is base (4daabb6) 97.14% compared to head (fb60c9a) 31.25%.

Files	Patch %	Lines
src/char-values.jl	0.00%	76 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #17       +/-   ##
===========================================
- Coverage   97.14%   31.25%   -65.90%     
===========================================
  Files           1        2        +1     
  Lines          35      112       +77     
===========================================
+ Hits           34       35        +1     
- Misses          1       77       +76     

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[gribeill]

Thank you for this useful PR!

[mahdi-sasar]

Thanks for the merge. Hope this is useful for everyone.
One question though: I am not familiar with the Codecov error that was generated. Can I help improve this somehow?

Codecov Report

Attention: 76 lines in your changes are missing coverage. Please review.

Comparison is base (4daabb6) 97.14% compared to head (fb60c9a) 31.25%.

Files Patch % Lines
src/char-values.jl 0.00% 76 Missing ⚠️
Additional details and impacted files
☔ View full report in Codecov by Sentry. 📢 Have feedback on the report? Share it here.