In this test, glaciers:
- increase to 100% over 4 days
- then stay constant for 4 days
- then decrease to 0% over 4 days

This also includes some extra bgc fields on the history file, because this test
is meant to be used to test bgc evolution as glaciers grow and shrink.

This also ensures that spun-up initial conditions are used, in order to provide a more
reasonable test of BGC state variable evolution with dynamic landunits. With a
cold start run, the state variables are evolving so fast that it's hard to see
the effects of dynamic landunits.
