make igraphalg_methods work for parametric types

[mxhbl]

igraphalg_methods only works for methods that have no parametric types in their signature. This is not a problem right now, but this means that the function may fail in the future. This PR should fix that.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 5.28%. Comparing base (618f384) to head (af2ad86).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/graph_api_extensions.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           master     #39      +/-   ##
=========================================
- Coverage    5.29%   5.28%   -0.01%     
=========================================
  Files           8       8              
  Lines        4310    4311       +1     
=========================================
  Hits          228     228              
- Misses       4082    4083       +1     

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[Krastanov]

Thank you!