Allows property updates to a model via command line

command line format is:

--property_change="PopulationName:PropertyName:value_with_dimension"
or
-p "PopulationName:PropertyName:value_with_dimension"

where value_with_dimension might be 34.3ms or 25mV.
Move utilities from Experiment into separate class
Moves some more small bits of code into the Util class