Implement check to see if custom beam and source surface have same shape

[arendMoerman]

Currently, PyPO does not raise an exception when a custom beam is assigned to a source surface with a different shape. I think we should implement a check that either

• raises an exception when an attempt is made to associate a custom beam pattern to a surface with different shape
• gives a warning and reshapes/resizes the surface to match the shape of custom beam pattern.

I guess the last one is what would be the nicest. However, the first option has the nice effect of really reminding users what went wrong. Something to discuss...