Document `RasterMetricSpace` and find faster implementation post-SciPy 1.13

[rhugonnet]

Raises:
E AttributeError: 'dok_matrix' object has no attribute '_update'

It was actually expected. At this line, I had made a comment that it was only a temporary solution to make the computation faster: https://github.com/mmaelicke/scikit-gstat/blob/main/skgstat/MetricSpace.py#L783
I'll take some time soon to look up if a fast solution is still possible with the latest release without calling a non-public method, otherwise uncomment the slower solution above.

Maybe it's the occasion to also streamline this RasterMetricSpace and add some documentation on it. It was a bit of a first draft when I first added it.

[mmaelicke]

Sounds good to me, thanks! Let me know if there is anything you want me to review.

[rhugonnet]

Errors due to SciPy 1.13 fixed by using the slower method in #185 (see #186).
Method dok_matrix.update still does not work, but there might be a new solution to keep the faster method, will look into it...