lattice.py split into files

[sbisw002]

@Ericgig I addressed the issues raised last time.

1. Split the lattice.py into multiple files.
2. removed duplicate stuff you pointed out.

It works with both qutip 4 and 5, since it is very independent actually.

I am working on a write-up to explain more.
Lattice1d is for single particle physics only.
The other classes like Lattice1d_fermi_hubbard or Lattice_1d_bose_hubbard are for multi particle physics. The user will only use one of them in their work, like there would not be any mix and match. The only commonality is they are both lattices. But all models are very disparate and user would only import one and work exclusively in one. More details to follow in the write-up.

Example usage can be found at https://github.com/sbisw002/qutip-lattice-notebooks/tree/master

I hope qutip-5 is on track!

[Ericgig]

@sbisw002
Thank you for the PR, but expect this to take a lot of time to review.
The PR is quite large and once merged, the qutip team will have to support it and I don't think we can right now.
Clements was telling me you were in an hurry, in that case you may want to publish it on your own and not wait for us.

[sbisw002]

Happy New Year. @Ericgig Thanks for the work. I will wait. Please let me know if I can help. I want to help with the maintenance as well.

[sbisw002]

@Ericgig , a reminder for this one!

[Ericgig]

I won't be reviewing this:

• I am not familiar with the physic to comment on it.
• For the code part, many of the issues in qutip_lattice is a separate repo(from qutip) #7 and before are still there, so I would just end repeating myself:
  • Many layer of for, if.
  • Test to hard coded values that seems to have been obtained from the tested functions.
  • Little code reuse.

Does this belong in qutip?
It works with numpy arrays more than qutip's Data and Qobj and re-implement qutip's functions. The coding style it different. To me it seems to use Qutip, but not be part of it.