First proposal for the MixedMapper and MixedOp

[Anthony-Gandon]

Summary

Fix #795

This draft PR introduces a MixedOp for manipulating systems corresponding to various "local" Hilbert spaces.
An implementation of the MixedMapper for mapping this operator into a sum of qubit operators acting on the composed registers is also proposed.

Updated tutorial
MixedOpTutorial.zip

[CLAassistant]

All committers have signed the CLA.

[Anthony-Gandon]

Update on the status of this PR.

• The MixedOp was implemented and will probably not change much
• The MixedMapper was implemented for the typical application to 1 system of Fermions coupled with 1 system of bosons. I still have in mind a generalization to allow for multiple fermionic subsystems.
  See images for the two ways to think of the problem. The first way assumes that we don't need to antisymmetrize the two subsystems
  

[coveralls]

Pull Request Test Coverage Report for Build 14032193908

Details

• 88 of 100 (88.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 85.995%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit_nature/second_q/mappers/mixed_mapper.py	43	44	97.73%
qiskit_nature/second_q/operators/mixed_op.py	43	54	79.63%

Totals
Change from base Build 13920710812:	0.04%
Covered Lines:	8873
Relevant Lines:	10318

---

💛 - Coveralls

[ftroisi]

Looks good overall, thanks!
I mostly commented on the docstrings

[fpietra]

Looks good, just a couple of very small comments.
Once they're addressed either positively or negatively, it looks good to merge.

[ftroisi]

Thank you @Anthony-Gandon for this valuable contribution!