Add quantitative morphology measures to Image

[stephenmwilkins]

Add ability to measure quantitative morphology to Image. E.g.

• half-light radius
• simple Sersic fitting
• CAS + Gini parameters

[WillJRoper]

I'm not sure exactly the best way to approach this. The initial approach that jumps out is methods on the images themselves but I'm not sure if we would actually rather split this off into another submodule for analysis?

[stephenmwilkins]

I'd prefer a method (or set of methods) on the Image base class. These could be elsewhere and inherited but I don't see why that would confer an advantage. Once generated they should also ideally be attached to the Image object too. For example,

CAS = image.get_CAS() # returns dictionary of CAS parameters

The get_CAS method itself would be a wrapper around methods calculating the individual parameters.