[Review]: 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.

The docstring should say that a `ValueError` (and not just "an error") will be thrown in case of a failure. Otherwise, users would have to play some try&error until they find out they have to catch `ValueError` in case of an error and even then they are not sure if other exceptions might be thrown by this method as well.