v2: merge .core and .problem (#414)

Move `v2.problem.*` to `v2.core`. Keep all pydantic models in the same module. Avoid unnecessary import complexity.

Author: dweindl

petab/v2/__init__.py

@@ -44,4 +44,3 @@
 )
 from .lint import lint_problem  # noqa: F401, E402
 from .models import MODEL_TYPE_PYSB, MODEL_TYPE_SBML, Model  # noqa: F401, E402
-from .problem import Problem, ProblemConfig  # noqa: F401, E402

petab/v2/converters.py

@@ -14,10 +14,10 @@
     ConditionTable,
     Experiment,
     ExperimentPeriod,
+    Problem,
 )
 from .models._sbml_utils import add_sbml_parameter, check
 from .models.sbml_model import SbmlModel
-from .problem import Problem
 
 __all__ = ["ExperimentsToEventsConverter"]