[WIP] symbolic algebra functionality

[lkdvos]

This is a very much WIP PR that starts showing some of the work on symbolic operator algebras.

Here, the main focus is on laying some ground work for two specific algebra types:

• a local algebra: this is supposed to be a symbolic version of local operators: operators that act on a local hilbert space (or a number of copies thereof), ie without knowledge of the geometry of the sites it is acting on.
• a global algebra: this is supposed to be a symbolic version of operators acting on the full hilbert space, ie with knowledge of the lattice etc. Here, operators are assumed to be acting non-trivially on a subset of the sites, and the identities can be left implicit.

To do

Build local operator types

• operators, sums, products, kron, functions, ...
• linear algebra
• Base.show
• Compatibility with SymbolicUtils.rewrite
• Define simplification rewriting rules

Global operator types

• Build global operator types
• Minimize code duplication between the two
• Testing
• Benchmarking