untabify
First draft of FlowVariables class to encapsulate variable handling
for velocity and pressure (#47)
todo
Have incompressible Navier-Stokes classes use FlowVariables class.
FlowVariables -> PrimitiveFlowVariables
HeatTransfer using FlowVariables
First draft of PrimitiveTempVariables.
HeatTransfer classes using PrimitiveTempVariables.
Boussinesq classes using Primitive*Variables now.
HeatTransferSource using PrimitiveTempVariables.