Add hierarchy to neuronal populations --tbe--

From the client:
_Each neuron type should be able to be tagged with both a cardinal class and a hierarchy of subtypes. We suggest that this be encoded by a string with a delimiter not used as a marker name (such as “@”). For example, a neuron with the key “V1” would be part of the V1 population while a neuron with the key “V1@MafA” would be part of both the V1 population and the MafA
subtype of V1.
On the website, the cardinal classes will form top level folders that can be toggled and recolored together. If they have subtypes, there should be a button to expand or hide the list of subtypes where the user can control each of them independently._

So the expects in this view to have a hierarchy in the population names. For example V2 would be the type, and 'a' would be the subtype.

I communicated the client this will take a while and that we will need to change the design, the FE, the data structure as well as agree on a new data format for the files as the information about population and sub population needs to come from the input file.