Closes #268.
My attempt to rename -force_to_thk. See #268.
Add the m_ prefix to PSForceThickness data members.
Fix installation of the installation manual.
Allow using a different ftt alpha in "ice-free" areas.

Use -force_to_thickness_ice_free_thickness_threshold to set the
threshold (in m) and -force_to_thickness_ice_free_alpha_factor to set
the scaling factor $C$ to use ($\alpha_{ice free} = C \alpha$).
Remove the "seconds_per_year" config. parameter.
Fix the -skip mechanism (again).

I think it is fixed, anyway.

I broke it in 'dev' a little while ago. (Luckily this is one PISM
feature that can be broken without negatively affecting run results.)
Add notes documenting SSAFEM residual and Jacobian computations.
Start switching to variables that actually mean something.
Re-write SSAFEM Jacobian and residual code.

The current version is less cryptic and closely follows the notes I
committed earlier today. And it still works! (I was surprised.)
Improve SSA FEM notes.
Merge branch 'dev' into blatter