Generalize Advection

[gutmann]

Permit advection algorithm to be specified separately for each advected scalar (e.g. upwind for a/b, MPDATA w/o FCT for c and MPDATA w/FCT for d...) and make Advection code loop through a list instead of specifying each variable by hand to permit arbitrary new species to be advected.

[gutmann]

there is a list of variables to advect now... but the algorithm can't be specified per variable. This may end up being a 3.0 or later feature