Make datacube an optional dependency of dea-tools

[robbibt]

Many functions in DEA Tools don't directly require datacube access (e.g. our coastal tide modelling functions, RGB and animation code, band indices, vectorising/rasterising tools etc). However, datacube is currently listed as a required dependency here:
https://github.com/GeoscienceAustralia/dea-notebooks/blob/develop/Tools/setup.py#L37

To make dea-tools easier to install, we should consider making datacube an optional dependency, using "extras" here:
https://github.com/GeoscienceAustralia/dea-notebooks/blob/develop/Tools/setup.py#L68-L72

This should allow users to install dea-tools without having to also install datacube.

[robbibt]

We should however ensure that some important dependencies like odc-geo and odc-stac are correctly included in our setup.py.