Startpoint sampling for PetabImporter-derived problems didn't work correctly
in case parameters were fixed in addition to those marked estimate=0 in the
underlying PEtab problem.

Fixing any parameters after the construction of the pypesto.Problem and the
corresponding startpoint method would lead to errors in during startpoint
sampling, because the list of fixed parameters was never updated.

In order to fix that, we need to have the current pypesto.Problem available
for startpoint sampling to get access to the fixed parameters.
Accessing `pypesto.Problem` is not compatible with the current
`FunctionStartpoints`, therefore we derive a new `PetabStartpoints` class
from `CheckedStartpoints.sample`.

Co-authored-by: Fabian Fröhlich <fabian@schaluck.com>