Numpy conversion consolidation function

[felix-e-h-p]

Consolidate numpy sample conversion into a single function - as referenced in #368 (think also related to #255)

Implemented single convert_to_numpy_sample(sample, t0) in convert.py:

• Accepts dict of xarray objects that already exist in PVNet Dataset classes.
• Modalities optional.
• The {X}SampleKey classes all removed.

All related tests updated to use string keys instead of now old class attributes.

[dfulu]

@felix-e-h-p this looks really good. Really nice to get rid of those SampleKey classes.

I've left a couple of comments. On the t0_idx I think we should strip it all out except for on the generation data. The original pipeline wasn't using those so I think we should just remove support for the NWP and sat t0_idxs. In the original we were just adding {"t0_idx": self.t0_idx} to each sample and I think we should just stick to that