Implementing alternative basis interface and abstract type changes in #40

[akirakyle]

This implements the alternative basis interface for dealing with tensor bases described in this comment and copied here:

shape(b::CompositeBasis) = b.shape
dimension(b::CompositeBasis) = prod(b.shape)
Base.length(b::CompositeBasis) = length(b.bases)
Base.getindex(b::CompositeBasis, i) = getindex(b.bases, i)

shape(b::Basis) = [length(b)]
dimension(b::B) where B<:Basis # must be implemented and should return Hilbert space dimension
Base.length(b::Basis) = 1
Base.getindex(b::Basis, i) = i==1 ? b : throw(BoundsError("attempted to access a nonexistent subsystem basis"))

[codecov]

Codecov Report

Attention: Patch coverage is 17.75362% with 227 lines in your changes missing coverage. Please review.

Project coverage is 27.22%. Comparing base (f7aaafa) to head (3e05d9f).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/bases.jl	37.69%	81 Missing ⚠️
src/superoperators.jl	0.00%	44 Missing ⚠️
src/deprecated.jl	0.00%	41 Missing ⚠️
src/julia_base.jl	0.00%	27 Missing ⚠️
src/embed_permute.jl	0.00%	20 Missing ⚠️
src/expect_variance.jl	0.00%	11 Missing ⚠️
src/abstract_types.jl	0.00%	2 Missing ⚠️
src/julia_linalg.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
- Coverage   34.66%   27.22%   -7.45%     
==========================================
  Files          15       15              
  Lines         424      551     +127     
==========================================
+ Hits          147      150       +3     
- Misses        277      401     +124     

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