Support extracting common subexpressions in `sx0` and `sx0_fixedParameters`

[dweindl]

For models with complex expressions for initial conditions sx0.cpp can become quite large and take hours to compile. Extracting common subexpressions is not implemented there yet and would probably reduce compilation times substantially. Implement that.

Avoiding substitution of w expressions into x0 would be even better, but would probably require substantially more changes.