Extend and improve solver interface

[tcunis]

All solvers in CaΣoS have three major interfaces:

1. Constructor (problem definition, options)
2. Function call (input: parameters, bounds, etc.; output: solutions)
3. Structure with statistics (S.stats)

CasADi also has a separate info struct (S.info) which we could implement, too.

The issue now is to decide (and implement) which additional information should be part of which interface (if any).

Addendum 2025-11-15:

A fourth interface, the handling of exceptions and display of error messages, should not be neglected either. See also #98.

[Fabian-Geyer]

S.info currently has the following structure:

• S.info.gram.Q.x → All Gram matrices (decision variables) stacked in vector format
• S.info.gram.Q.g → All Gram matrices (constraints) stacked in vector format

Similarly:

• S.info.gram.dim.x / g → Corresponding dimensions of the Gram matrices as vectors

Idea: Would it be reasonable to return the list of Gram matrices already reshaped into matrix format? This could potentially be more convenient for usage.