Add the "PISMFEvoR" class.

Compiles and runs (and does nothing). Needs testing, of course.

Try

pisms -y 1000 -o foo.nc
pismr -i foo.nc -stress_balance sia_fevor -o bar.nc

bar.nc should be the same as baz.nc produced by

pismr -i foo.nc -stress_balance sia -o baz.nc

Next up: think about initializing this thing. We need to figure out what
"state variables" we have, i.e. what we need to read at the beginning of
the run (and what to save at the end). Also: do we need some
"bootstrapping" code?