Perhaps control old/new style with minimum_allowed height
	
Test friction term!

	
Create a pyvolution_interfaces.py module with all getters/setters
converts etc.


gradient should not require qs to be numeric arrays anymore.!!


Let rectangular mesh and others 
like it to generat the three lists for use when instantiating
Mesh and DOmain objects.


Don't use default conserved qs - hardwire into shallow_water.py


Interface

 set_quantity should be ablt take either a function or an array.
 

Design:  
explicit and implicit updates should be stored in each quantity. 

Create repr of a domain such that it lists all its parameters e.g. order