Check data array passed mosaic.polypcolor matches dimension of descriptor.ds

[andrewdnolan]

As demonstrated in #20, there currently isn't any warning or error raised when the dimension of the data array passed to mosaic.polypcolor does not match the dimensions of the dataset used to create the mosaic.descriptor.

This is a common failure case, when a user creates a descriptor with a "full" dataset but (accidentally) passes a regionally sub-setted data array to mosaic.polypcolor. This will cause incorrect indexing issues, even through it won't cause an out of bounds error.