Initial commit: Get the unique id of each parameter in a lensing entity.

[gvernard]

We need a function to be called on each 'lensing entity' that returns the unique ids of its parameters. We will use these parameter ids to match them with the columns of the chain file. We could give a list of parameter names to this function (just the profile parameter name, e.g. 'q' for the axis ratio). If no arguments are given then all the parameter ids should be returned.

[aymgal]

@gvernard I just added a new method to the LensingEntityList class that allows to retrieve the list of IDs with something like coolest.lensing_entities.get_parameter_ids(with_name=None). Let me know if this is what you needed.
You have an example usage in this notebook.

[aymgal]

@gvernard Please let me know if the new function is what you were looking for so I can merge the PR