Implement till friction angle optimization

[ckhroulev]

Implements a till friction angle optimization scheme. This PR replaces #410, which was based on an old PISM version.

To test:

• create a mask specifying locations where ice thickness is held constant:

  ncap2 -O -s "where(topg < 0 && thk*(910.0/1028.0) < 0 - topg) thk_bc_mask=1;" in.nc out.nc

• Set -geometry.update.use_basal_melt_rate false to disable geometry changes due to basal melt.
• Add no_gl_retreat to "surface model modifiers", i.e. use -surface simple,no_gl_retreat or similar.

See doc/sphinx/manual/modeling-choices/subglacier/basal-strength.rst for more details.

Checklist

• [x] update documentation
• [x] update CHANGES.rst
• [ ] ~add regression tests~

[talbrecht]

Wow, this is great!