Selective updating on catalog parameters

[syasini]

Before running build_dataframe() check to see if parameters already exist in the catalog.

At the moment all parameters are calculated upon instantiation which causes large catalogs to load slowly.

[syasini]

An idea would be to add a new argument to Catalog to indicate which variable should be calculated on instantiation.

for example

catalog = Catalog(data, calculate_vars = ["R_200c", "c_200c", "v_r", "v_lon", "v_lat"])

would only calculate the given variables in build_dataframe.

This could be implemented by adding a hidden variable self._calculate_vars in Catalog and bunch of statements like

if "R_200c" in self._calculate_vars:

to build_dataframe.

This is perhaps not the best idea so feel free to suggest better ones!