add sympy conversion

[jonas-eschle]

As an idea, we could add conversion to sympy. This would allow to use the full power, including:

1. latex formatting
2. simplification of expression
3. resolve Remove unneeded parentheses in output strings #2 by converting forth and back as sympy removes redundant brackets automatically
4. functional form with lambdify allowing to use "arbitrary" backend

The implementation should be straight forward:

1. create all variables as symbols and inject in future sympify calls
2. convert an expression to numexpr and sympify it

The only caveat I see so far is that the == operator has a different meaning in Sympy and is not a logical equal (https://docs.sympy.org/latest/gotchas.html#double-equals-signs), furthermore booleans and numericals don't mix so implicitly.

So it seems to me an interesting idea where we may gain a lot, but it could also be a nightmare if the differences in behavior are too large. I don't know Sympy well at all, so maybe others have an opinion? What are your estimates?