changes from nbdev framework

[thomasdkelly]

The nbdev framework was a really useful template when starting this library from scratch, however there are certain changes we would like to make (e.g., ahead of time compling) that cannot currently be incorperated into the 'notebooks -->.py' workflow.

• Currently the module code is wriiten in the /nbs/ folder which is exported to the /aquacrop/ folder. Changes should now be made to the .py files themselves making the /nbs/ folder redundant
• The /nbs/ folder could instead be used for tests
• The docs are currently generated from the notebooks so this will have to be changed (maybe using https://squidfunk.github.io/mkdocs-material/)

[thomasdkelly]

After recent changes to docs and github action workflows e.g., #27 , no longer tied to nbdev. Will delete /nbs/ directory in future