Changes in IceModelVel2T (needs code review).

I think this is needed to allow for correct air_temp_sd reporting in the constant case. Previously it would fail with segmentation fault. I am a bit worried about introducing changes into PISM internals this way, so please double-check my changes.

* New function IceModelVec2T::init_constant to initialize as constant in time and space but ensure that the time coordinates and time bounds are defined. Or should this be a re-implementation of the set function instead?
* IceModelVec2T::average does nothing if the time dimension has length one. Without this, the averaging fails in init_interpolation, when deciding whether to interpolate to the right.

Now air_temp_sd should be reported correctly in both constant and variable cases. This should help with #179.