[Emu-SV] Monkey patch and qubit density sparse apply

[murogrande]

Related to issue #7
Monkey patching the apply method of Qubit Density callback. The custom apply is "sparse" implementation of the following operator:

$I \otimes I \otimes n_i\otimes ... ,$
where $n_i$ is the number operator

New/updates

• emu_sv folder: custom_applies.py script where I will store the custom apply methods
• test: emu-mps: I added a test related to qubit_density callback. The same is emu-sv. Both results should be same upto certain tolerance

[a-quelle]

Apart from my comments on the code, I have an additional question:
Do you want to do the correlation matrix and energy related observables in this MR also, or in a separate one? The approach you take here is good, so doing the others will be kind of the same.

[murogrande]

Apart from my comments on the code, I have an additional question: Do you want to do the correlation matrix and energy related observables in this MR also, or in a separate one? The approach you take here is good, so doing the others will be kind of the same.

I will do it in a separate MR. I prefere having a small MR that is easy to review