change default site value for FiniteMPS in entanglement_spectrum

[VictorVanthilt]

As pointed out by @borisdevos, calling entanglement_spectrum(::FiniteMPS) does not work as the default value for site is 0, which would call the C tensor left of site 0, which does exist, it is just a trivial 1, but throws a BoundsError due to the checkbounds in entanglement_spectrum.

Currently I just split off the entanglement_spectrum method that dispatched on FiniteMPS to have a default site value of 1. This works but now the docstring is not consistent as it shows a default value of 0 for both methods.

In my opinion having a default value for the finite case really does not make sense and the user should just choose the site according to their needs.

Any opinions?

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/algorithms/toolbox.jl	75.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/algorithms/toolbox.jl	94.78% <75.00%> (-1.37%)	⬇️

... and 2 files with indirect coverage changes

[lkdvos]

This is definitely ok for me, and removing the default value for the FiniteMPS also seems reasonable.
It might also be a good idea to check if we are using site as a kwarg or a regular argument in other parts of the code, at least for me I know that this often confuses me

[VictorVanthilt]

Ok, then I will change it to no default argument for FiniteMPS and I will try to reflect it in the docstring.

[VictorVanthilt]

@lkdvos What do you think of this implementation? (The one I just pushed.)