CMEnergies as float_range involves passing filters around a lot

[alisonrclarke]

Code is currently on my fork, but the initial fixes for #143 involve passing the currently selected filters around a lot, e.g. to add the aggregations in es_config.py, and when parsing the aggregations in aggregations.py.

Part of the reason for this is that support for ranges in elasticsearch is not (yet) as comprehensive as support for float types: see elastic/elasticsearch#34644

One potential option would be to see if we can use painless scripting in elasticsearch to so some of the work for us.

We need to do the migration soon so I'm not going to spend any more time on the issue now, but am logging it as something to look into later on if we have time.