Added checks for the compatibility of the solvers

[Matskuu]

Defined enumerations for variable dimensions: scalar, vector or tensor. Vector means 1D tensors and are only relevant for polars evaluator at the moment.

Also implemented a function to check for compatible solvers for a given problem. It checks the problem's linearity, differentiability and the highest dimension of the decision variables. Based on these checks, the function returns a list of compatible solvers for the problem in question. A downside to this is that for this to be accurate, all the objectives and constraints have to have explicitly defined whether they are linear and/or twice differentiable.