OpSum to TTN conversion feature completeness

[mtfishman]

OpSum to TTN conversion (spearheaded by @leburgel) needs some more testing and improvements to be considered "complete" (i.e. have all of the same features as the OpSum to MPO conversion in ITensors.jl). Here is a summary of the status thanks to @leburgel:

The default current default implementation should work for any long range interactions (at least for anything I've tried so far), the only problem is that the resulting network has a bond dimension that is larger than necessary. There is an issue when I manually compress the network by merging interaction channels (similar to what is done in ITensors.jl) that I haven't figured out, so I've just switched off the compression by default for now until this is fixed. So right now everything should work correctly as is, it's just not optimal. I should have time to try and fix this soon though.

Everything with QN conservation errors by default right now I think. For fermions, there's definitely things in the converter that are just plain broken. It's hard for me to tell how easily this can be fixed though, since I'm not really familiar with how ITensors.jl handles fermions. For regular symmetries however, I don't think there should be any fundamental issues. I've just never tested any of the tree tensor network code with QN number support so this errors more as a precaution. It should be a matter of trying it and seeing whether things break at some point. I can also try to have a look at this soon.

Originally posted by @leburgel in #95 (comment)

@b-kloss was starting to look into this as well, we should make sure to coordinate efforts and also use this issue to compile a list of bugs we come across.

[mtfishman]

I believe this is completed by #104, #116, and #122.