[ENH]: add mask to Info object

In order for the regridding algorithms to deal with masks, we need to be able to exchange mask info as meta data.

Info object will get a mask property and input argument.

We want to have the following options:

• flex_mask: The current behavior should be still possible and will be the default
• unmasked: only plain numpy arrays are exchanged
• valid mask values from numpy.ma sub-package (Either bool-array or special nomask value): exchanged array must be a numpy.ma.MaskedArray

The flex_mask and unmasked constants should be provided by finam.

The regridder can then use the (constant) mask to setup their algorithms.

A masking adapter should be implemented to apply a mask to a correctly shaped array.