Support for discrete noise distributions

After https://github.com/PEtab-dev/PEtab/pull/654.

• Extend distribution enums
• Add new classes to distributions.py
• Add continuous/discrete flag to Observable and Distribution.
• Validation for integer measurements of discrete observables

Begin by reviewing the changes in PEtab PR 654 and the existing distribution enums and distributions.py. Trace how Observable and Distribution currently represent distributions, then determine where integer-measurement validation belongs. Done means discrete distributions and classes are supported, both objects expose the continuous/discrete distinction, and integer measurements are validated.