Started work on refactoring colormap creation.  Move some functionality
from Satpy to here.  Deprecate passing a string containing a CSV to
``Colormap.from_file``, this should now use ``Colormap.from_string``
instead.

Improve documentation in colormap class and rewrite
from_sequence_of_colors in terms of other methods available.

Refactor from_xrda and rename it from_array_with_metadata.  This is the
method with somewhat funny behaviour that was copied from
satpy.composites.ColormapCompositor.build_colormap.
It includes some behaviour that I cannot explain, such as cutting off
the last value of the colour palette when meanings are not provided.
I copied this behaviour as-is from Satpy.

Add a flag to disable the behaviour to remove the last value from the
colormap/values.

Fixxing one merge conflict.

Moved the test-cmap-from-string test method out of the class and changed
it into its own function, because it doesn't read from a file.