Add requirements.txt file to collate the dependencies

[surbhigoel77]

Closes #1

[ma595]

Please check if xarray[netcdf4] is needed, or if pip install netcdf4 will suffice.

We also need a version added to scikit-learn.

A frozen-requirements.txt is provided in the notes repo.

[ma595]

@surbhigoel77, can you please confirm that the requirements install successfully and you have executed at least the main_forcast.py file.