Implement NeuroML 2 version of calcium pool mechanism

[pgleeson]

Need to make custom NML2 implementation which behaves identically to mod file impl:
https://github.com/OpenSourceBrain/L5bPyrCellHayEtAl2011/blob/master/neuroConstruct/cellMechanisms/CaDynamics_E2_init_mod/CaDynamics_E2.mod

[pgleeson]

Can reuse custom Ca pool example from:

https://github.com/OpenSourceBrain/FarinellaEtAl_NMDAspikes/blob/master/neuroConstruct/cellMechanisms/cad2_NML2/cad2_NML2.nml

[pgleeson]

Working nml2 version of this channel is at https://github.com/OpenSourceBrain/L5bPyrCellHayEtAl2011/blob/master/neuroConstruct/cellMechanisms/CaDynamics_E2_NML2/CaDynamics_E2_NML2.nml

[pgleeson]

There remains the issue that the parameters decay & gamma are altered when this mechanism is placed on the soma/apical dendrites of the full L5PC cell. This is not yet correctly exported to NML2 when the cell morphology is exported.

Related to LEMS/jLEMS#32