DataGrid DataPoint DataBlob Inheritance

[benjaminleighton]

@pbranson noticed that DataPoint (and thus indirectly DataGrid) inherit from DataBlob and we are unsure why this is the case. He has fixed this in https://github.com/rom-py/rompy/pull/159/files however there are some other changes there I am reviewing but if you @rafa-guedes or @tomdurrant could take a look specifically at the inheritance issue that would be good.

[rafa-guedes]

True, they don't really need to inherit from DataBlob - I suggest we define a base, abstract class DataBase just defining the common fields (model_type / id) and the basic api (the get method), and used it with both DataBlob and DataPoint. Would that work?

[rafa-guedes]

@benjaminleighton I have removed the dependency of DataPoint/Grid on DataBlob, and added a new abstract base class that is very simple and just defines common fields and the basic get signature, in #164.