Functions to get information out of a MapCube.

I added in three functions:
_maps_have_same_number_of_x_and_y_pixels 
 - tests if all the maps have the same number of x and y pixels

data()
 - takes the image data in a mapcube and returns a 3d numpy array

meta()
 - returns the meta objects from each map in a list

These functions are handy when working with mapcubes.  Often when you have done what you have to do with your mapcubes, you just want the information out (particularly the image data).  These functions make it easy to do that.  If these functions are acceptable I'll work on the documentation and testing.

Ok, so firstly, properties are functions that are accessed like attributes, so no they do not take up RAM.

I don't think we should have `.data` and `.meta` for mapcube in the way you have defined the functions, as they are not the actual underlying data and metadata for the `MapCube`. So I think having the functions you have defined as helper methods is great, but we shouldn't confuse where the actual data source is with re-using the same API as the other datatypes but them not having the same meaning.