Changing Parcels -> parcels and importing all of parcels throughout

[erikvansebille]

Most python packages are all-lowercase (as suggested by pep8), so we should also change the writing of parcels to lowercase throughout in the examples, documentation, docstrings and website.

While we're at it, we could also change the notebooks in the examples to refer to the classes as parcels.ClassName, to better highlight that these are specific to parcels. So e.g. change

from parcels import FieldSet

fset = FieldSet(...)

to

import parcels

fset = parcels.FieldSet(...)