Bugfix: #21 - Explicitly use h5netcdf engine in xr.open_dataset()

[kaedonkers]

Fixes #21

Changes

• Add engine="h5netcdf" to xr.open_dataset() in __init__.py files for opening .nc data
• Add hdf5 and cftime explicitly in dependencies

NOTICE

Please test this before merging, I'm getting some weird behaviour on MacOS and don't have a Linux machine to test it on

[jucordero]

I believe the actual dependency should be h5py instead of hdf5
When changing it I had no trouble with the new import engine so should be fine.

In any case, after a long period of neglect I'll come back to this repo and will include proper documentation and tests, including import tests on different architectures.