Only compute the viscosity if a strain rate was given. This avoids
accessing empty arrays and also matches the documented behavior.
Add a testcase.
Add an entry to changes.h.
Merge pull request #45 from bangerth/why-evaluate-viscosity-in-adiabatic-conditions

Why evaluate viscosity in adiabatic conditions
Evaluate time dependent boundary conditions in every time step, in just the same way as we do for the velocity boundary conditions.
Add to the documentation.
Add a changelog entry.
Adjust a couple of comments.
Merge pull request #44 from bangerth/time-dep-bc

Time dep bc
Fix the description of a parameter.
Use introspection in several more places. Also rename AdvectionField object 'advf' rather than 'torc' for clarity.
Merge pull request #48 from bangerth/fixes

Fix the description of a parameter.
Merge remote-tracking branch 'geodynamics/master' into introspection_patch
Remove a TODO.
Compacted one of the lines to more closely match the way it was in geodynamics/master
Again, minimizing differences between this and geodyamics/master
Temperature boundary condition update

Add an update() function in temperature boundary conditions.
Call this function when time step start in core.cc
Use introspection in even more places.
Simplify a few introspection queries, remove some TODOs
Missed one