Generate python code for schemas directly from Julia

[deltamarnix]

Remove intermittent json generation and dependency on the data model code generator

Fixes #904
Fixes #1077
Fixes #887

[visr]

This looks like a good improvement, thanks!
The error message from #904 has gotten a lot better, pointing at the actual problem:

>>> import ribasim
>>> import pandas as pd
>>> ribasim.Terminal(static=pd.DataFrame(data={"no_id":[1]}))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "D:\Ribasim\.pixi\env\Lib\site-packages\pydantic\main.py", line 171, in __init__
    self.__pydantic_validator__.validate_python(data, self_instance=self)
pydantic_core._pydantic_core.ValidationError: 1 validation error for Terminal
static.df
  Value error, column 'node_id' in DataFrameSchema {'node_id': <Schema Column(name=node_id, type=DataType(int64))>} requires a default value when non-nullable add_missing_columns is enabled [type=value_error, input_value=   no_id
0      1, input_type=DataFrame]
    For further information visit https://errors.pydantic.dev/2.6/v/value_error

Still I wonder how many users will understand "requires a default value when non-nullable add_missing_columns is enabled", instead of something simply stating that a required column is missing.