Allow filename variable remapping
This MR adds the option to remap variable names inside files to different names in XML functions. For example, IncNavierStokesSolver
fields containing u,v,w,p
can be mapped to linear advection variables Vx,Vy,Vz
using the syntax:
<FUNCTION NAME="AdvectionVelocity">
<F VAR="Vx,Vy,Vz" VALUE="file.fld:u,v,w" />
</FUNCTION>