Refactor v2.*Tables (PEtab-dev#417)

DRY

Introduce `BaseTable` to implement common functionality of `{Observable,Parameter,...}Table`.

Author: dweindl

doc/modules.rst

@@ -37,5 +37,4 @@ API Reference
    petab.v2.experiments
    petab.v2.lint
    petab.v2.models
-   petab.v2.problem
    petab.v2.petab1to2

petab/v2/converters.py

@@ -401,7 +401,7 @@ def _add_indicators_to_conditions(self) -> None:
         #  removed. Only keep the conditions setting our indicators.
         problem.condition_tables = [
             ConditionTable(
-                conditions=[
+                [
                     condition
                     for condition in problem.conditions
                     if condition.id.startswith("_petab")