Extend Tenet.contract to contract :between two Sites

[jofrevalles]

Summary

This PR enhances the Tenet.contract function, enabling it to contract the singular values within a bond between two Sites. It introduces an optional direction argument, allowing users to specify whether the singular values should be contracted onto the :left or the :right tensor.

Additionally, we have added tests for this new functionality, and refactored the existing tests for the canonize! function, resulting in more compact and efficient test code.

Example:

Let's exemplify the function:

julia> qtn = MPS([rand(2, 2), rand(2, 2, 2), rand(2, 2, 2), rand(2, 2, 2), rand(2, 2)])
MPS (inputs=0, outputs=5)

julia> canonized = canonize(qtn)
MPS (inputs=0, outputs=5)

julia> length(tensors(canonized))
9

julia> contract!(canonized, :between, Site(1), Site(2))
MPS (inputs=0, outputs=5)

julia> length(tensors(canonized)) # We removed one singular value vector
8

julia> select(canonized, :between, Site(1), Site(2))
ERROR: ArgumentError: collection must be non-empty
Stacktrace: ...

julia> @test isapprox(contract(TensorNetwork(canonized)), contract(TensorNetwork(qtn)))
Test Passed

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.00%. Comparing base (11890f0) to head (dcfb136).
Report is 12 commits behind head on master.

Additional details and impacted files

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##           master      #30   +/-   ##
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  Coverage   75.00%   75.00%           
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  Files           8        8           
  Lines         436      460   +24     
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+ Hits          327      345   +18     
- Misses        109      115    +6     

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