Add py.typed to package root

[samjmolyneux]

PEP 561 and Python's Distributing Type Information state:

Package maintainers who wish to support type checking of their code MUST add a marker file named py.typed to their package supporting typing.

[willGraham01]

An going to lump #27 into this issue too, since we should do both at the same time when we come to it.

To surmise so everything is in one place, in addition to adding a py.typed file;

I think our plan for this, for now, should be to make a _typing module or something, and inside it create a few TypeVars or TypeAliases that cover the things we need to standardise.

Then go through the package and typehint accordingly - that should hopefully avoid us having to do this again some time in the future... It also lets us helpfully avoid commitment to a particular framework for now (though jax should probably be our default).

EG (imports / correct class names not promised)

_typing.py

RNGKey: TypeAlias = jax.Array | numpy.Generator
ArrayLike: TypeAlias = jax.Array | numpy.typing.ArrayLike

some_other_file.py

from causalprog._typing import RNGKey


def random_function(rng: RNGKey, ...):
   ...