Add str & unicode conversion methods to UnitLabel in Python.

Refs #9252
Add UnitLabel inequality operators plus tests

Refs #9252
Allow PlotAxis to use IMDDimension objects alone.

Also put the "Spectrum" label as a Label unit in SpectraAxis
Refs #9252
Harmonize creation of plot axis titles for MD & nonMD sources.

Refs #9252
Put back methods to return plain strings for unit labels in Python

The main Unit::label is deprecated and issues a warnings but scripts
won't break
Refs #9252