Finalise GLIFs

[pgleeson]

• Add metadata to GLIF_123.xml files
• Check through GLIFs.xml definitions and add notes
• Add OMV tests on a number of individually converted models (e.g. this) to test exact spike times
• Move to CellTypesDatabase
• Test all on Neuron (omv tests on jNeuroML_NEURON)
• (Possibly) add version of cell models with resting memb potential moved to correct value (not 0mV)
• Add GLIF Comp Type to standard NML2 cell models