Overload Base.write for pyramids

[felixcremer]

This allows to save a pyramid to disk by calling savecube for all layers of the pyramid.

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.83%. Comparing base (be4c984) to head (c1f4739).

Files with missing lines	Patch %	Lines
src/PyramidScheme.jl	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #92      +/-   ##
==========================================
+ Coverage   72.75%   72.83%   +0.07%     
==========================================
  Files           4        4              
  Lines         345      346       +1     
==========================================
+ Hits          251      252       +1     
  Misses         94       94              

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