Refactor SBML sympification

[dweindl]

At some point it would be convenient to replace the current SBML-sympification by https://github.com/dweindl/sbmlmath/.
This gives us better control over the whole process and can replace the awkward log and piecewise handling, handling of SBML constants and special functions. And the initial collection of all SBML symbols would become obsolete.
Not expecting a relevant speed gain, but improved maintainability.

[dweindl]

Furthermore, I think the current sympification is problematic because of differences in operator precedence between SBML L3 formula strings and sympy (which is based on Python rules) that are ignored in amici.

[dweindl]

The major part is addressed in #2681.

The spline-related code still uses some custom MathML printer and plain sympify. After addressing that on, more old code from sbml_utils.py can be removed.