OperatorList class to keep memory usage low when storing operators

Created by: VolodyaCO

This PR presents a data class to handle the matrix representation of operators when building local operators, so that we don't keep unnecessary copies of the same operators over and over.

For fermionic two- and four-body operators, the memory overhead of storing many times the same matrix representation of an operator is not very costly. However, for bosonic two- and four-body operators, the matrix representations can become very large, and therefore, a lot of memory might be consumed unnecessarily.

This issue is resolved with this OperatorList class which handles the attribute _operators from a LocalOperator. Before, the LocalOperator._operators attribute was a list, and now it's an object of OperatorList. Before, every operator was stored in the list, even though in most of the applications the operators are actually repeated because they have the same form. Now, operators are hashed, and a lookup table is implemented so that no repetition of the matrix representation of the operators are stored.

I haven't checked the code, but maybe something similar can be done for operators in LocalLiouvillian. This was particularly intended to create GraphOperators so that it does not store the same operator over and over. In this PR I have not modified the GraphOperator's logic. You tell me if you want me to do it here, or in other PRs, as I understand you want to have small PRs.