Add dataarray scatter

[Illviljan]

Splitting up #5622 as the scope of it has grown too large now.

• Adds support for dataarray scatter plots and replaces the dataset version.
• Scatter now has 3d support with the z argument.
• Scatter now always returns a single pathcollection, earlier it could return a list of pathcollection when using categoricals
• Better legend, handles now categoricals. Making continous/discrete options slightly redundant, still there though.
• Facetgrid generalized slightly to handle 3d plots.

• [x] Tests added
• [x] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [ ] New functions/methods are listed in api.rst