add jme 'pluralise' function

wraps Numbas.util.pluralise
allow new types to be added to treeToJME/texify

If you make a new JME data type, you can add code to turn it into TeX or JME by adding a function to Numbas.jme.display.typeToTeX or Numbas.jme.display.typeToJME. This replaces the old switch-statement method which couldn't be modified.

There's also now Numbas.util.equalityTests, which does the same thing but for checking two elements of the same type are equal.
fix jme function `factorise`

it wasn't returning wrapped numbers