Add Hay 2011/Blue Brain/Allen CaDynamics [Ca2+] pool as standard, recommended concentrationModel

[pgleeson]

Is your feature request related to a problem? Please describe.
The standard concentrationModel: https://docs.neuroml.org/Userdocs/Schemas/Cells.html#decayingpoolconcentrationmodel is really best just for single compartment cells, see #152 (comment). Other instances of this using custom LEMS are used for multicomp cells, e.g. here https://github.com/OpenSourceBrain/AllenInstituteNeuroML/blob/master/CellTypesDatabase/models/NeuroML2/CaDynamics.nml are more robust/better tested.

Describe the solution you'd like
Incorporate a modified/updated/more documented version of this https://github.com/OpenSourceBrain/AllenInstituteNeuroML/blob/master/CellTypesDatabase/models/NeuroML2/CaDynamics.nml to the core NML types and make it the recommended version