Astropythonize the core or not

[maray]

In my latest refactoring (yep... I do one each month it seems), I've removed the nddata and table support from the core. This was done because the core should be used by the UPI and Algorithms, so we can put all the Astropy data structures (Table and NDData) in these other packages rather than in the core. I have not done this for all the functions. Therefore,

• We must decide if we will follow this idea
• If yes, then things like example measure_shape(), that returns a astropy table, should we deferred to the UPI and Algorithm subpackages

[maray]

We decided to keep astropy away from the core (as much as possible)