SSAFEM: Move some computations out of inner loops.
Fix RACMO surface mass balance missing value issue.  Port to 0.6.

In the std-greenland example it was quite a big error to keep the
RACMO smb=0 fill value, which affected most runs in section 1 of the User's
Manual.  This fix is a reasonable choice instead: -1 m/a ablation where
smb is missing.
Fixes to figures in User Manual resulting from change in previous commit; figures 3--9 fixed.  Text updated.
Remove FEQuadrature::init().
Completion of revisions to first section of user manual, based on change in commit fc93b61ef.
Avoid passing classes by value. Close #269.

(Pass classes by "const reference" instead. This seemed like a fun thing
to do on a Saturday.)
Clean up indexing of Jacobian entries.
Don't include .py files in etags.
Improve FEQuadrature::getWeightedJacobian().
Close #270.
Tweak SSA FEM notes.
A better name for SSAFD CFBC notes.
Fix a file name and small edits in section 1 of User Manual.
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.
Improve code reuse in SSAFEM.

(I was making sure I understand how we enforce Dirichlet boundary
conditions.)
Merge branch 'dev' into blatter
Clean up the merge so that it compiles again.