Added dimensions to state variable objects and all xml writer methods
(i.e. for state variables, analog ports, etc)
Added support for piecewise mathinline expressions
Added support for initial_conditions for state_variables
Merge branch 'adding_dimension_attributes'
Merge branch 'adding_mathinline_piecewise'

Conflicts:
	lib9ml/python/nineml/abstraction_layer/dynamics/component/expressions.py
Merge branch 'adding_initial_states'