Add `py.typed` file in `/nowcasting_datamodel

[simlmx]

nowcasting_datamodel is typed (to some extent), but when used as a dependency, the types are ignored by mypy unless a py.typed file is provided (See https://peps.python.org/pep-0561/).

Context

We use nowcasting_datamodel as a dependency in pv-site-production and it would be nice for mypy to be able to use the types from nowcasting_datamodel.
It would be useful for any other package depending on nowcasting_datamodel that wants to use mypy.

Possible Implementation

Add file /nowcasting_datamodel/py.typed

Add

 package_data = {
        'nowcasting_datamodel': ['nowcasting_datamodel/py.typed'],
}

to /setup.py.

Example of implementation in pv-site-prediction but with poetry.