Improve interactions with SpineML(Creator)

[pgleeson]

NeuroML2/LEMS -> SpineML

Implementation here: https://github.com/NeuroML/org.neuroml.export/blob/development/src/main/java/org/neuroml/export/xineml/XineMLWriter.java

• Convert ^ to pow() in expressions

SpineML -> NeuroML2/LEMS

Being worked on by @ajc158...