Refactor LoopTNR

[Chenqitrg]

The current LoopTNR is too long. Therefore it might be helpful to refactor it. The LoopyMPS part can be viewed as a module and LoopTNR algorithm does not necessarily to know the detail of how the approximation is solved.

Moreover, the LoopyMPS part can be reused by other algorithms, such as LoopTNR algorithm on other lattices.

Here are three plans:

1. Create a folder called loop methods in schemes, and the loopymps is a file
2. Create a file called loopymps in utilities
3. Add the LoopyMPS to other packages and looptnr imports it

[VictorVanthilt]

If you do this I would suggest calling it PeriodicMPS and not LoopyMPS.
To which other packages would you add this?

[Chenqitrg]

Still not sure. Need to discuss with you guys. It is also possible to add it to utility. Or in the LoopTNR folder.
If separate the PeriodicMPS, I would suggest it to go MPSKit.