Make Distinct Field Filters Apply Correctly on ISIS Endpoints

[MRichards99]

This PR will close #114.

This branch was initially just meant for testing all the existing issues with the bug label, but I noticed #114 didn't work properly on the new backend so I fixed it on this branch.

I've fixed this by replacing the isis_endpoint parameter in ICATQuery with an if statement:
if distinct_attributes != []:

This ensures mapped_distinct_fields is always None on an ISIS specific endpoint (where a query aggregate of DISTINCT is required) but no distinct field filter is used. This then prevents the issue before isis_endpoint was implemented (a list of empty dictionaries as the response body). Removal of isis_endpoint means that when a distinct filter is used, code relating to that will be executed (and not blocked by and not self.isis_endpoint). Ordinary endpoints still work as they should.

On the ISIS dev ICAT, this request served as a good test:
http://{{datagateway-api}}/instruments/23/facilitycycles/84762458/investigations?distinct="id"

[louise-davies]

This fixes the issues I had with the other PR and it's working nicely (insofar as distinct filters work right now - perhaps I should rereview once distinct filters are fixed? although I doubt it will affect these changes)