Seperation of Dependencies

[HCookie]

What maintenance does this project need?

As we saw in #81, a bug/breaking change in one of our dependencies, can cripple the install of this package.

With #81, datashader is only needed in the plotting callbacks, this is not a critical component of training and ultimately should not be required dependency.

We should seperate these into optional dependencies and catch when a part of training is trying to use an uninstalled package.

# Optional Dependencies
[optional-dependencies]
plotting = ['datashader', 'matplotlib']
...

What other splits of dependencies make sense?

Organisation

ECMWF

[JPXKQX]

I think you can add "pyshtools>=4.10.4" to the plotting.