Fix building pyramids and add cat method

[felixcremer]

This fixes the building of Pyramids and improves the plotting functionality.

This also adds a special cat method for Pyramids so they are concatenated on al levels.

Fixes #74
Co-authored-by: Anshul Singhvi [email protected]

[codecov]

Codecov Report

❌ Patch coverage is 88.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.66%. Comparing base (aa6bff7) to head (fcd51fd).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/PyramidScheme.jl	85.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
+ Coverage   69.55%   70.66%   +1.11%     
==========================================
  Files           4        4              
  Lines         289      300      +11     
==========================================
+ Hits          201      212      +11     
  Misses         88       88              

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