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.

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