Implement copy for bases

[blegat]

This is needed to pass the tests of MultivariateBases: JuliaAlgebra/MultivariateBases.jl#57. It's not clear why it's needed though since SumOfSquares does not seem to use it:
https://github.com/jump-dev/SumOfSquares.jl/blob/2c40b50abcc31b89111f07d62c0c05752be9d8a1/src/sets.jl#L149
Let's not merge it for now, I'll check if that's really needed
EDIT used by SOS here:
https://github.com/jump-dev/SumOfSquares.jl/blob/ba0f7b09b2ad5a8b7e8eaef7acd040835d325bfb/src/Bridges/Constraint/sos_polynomial.jl#L65-L68

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.96%. Comparing base (dd6c877) to head (c736201).
⚠️ Report is 48 commits behind head on main.

Files with missing lines	Patch %	Lines
src/bases_dirac.jl	0.00%	1 Missing ⚠️
src/bases_fixed.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
- Coverage   84.15%   83.96%   -0.20%     
==========================================
  Files          15       15              
  Lines         871      873       +2     
==========================================
  Hits          733      733              
- Misses        138      140       +2     

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