Separate scalar diagnostics for each ice sheet + parameters to control ice-sheet velocities (#714)

* Updated with dev/gfdl. Then, added parameters for min ice thickness, min basal traction, max surf slope, and min ice viscosity to use for ice dynamics

* noted how the MIN_BASAL_TRACTION parameter input is in units [Pa m-1 yr], but converts automatically to [Pa m-1 s] in the code

* Added separate scalar ice-shelf diagnostics for Antarctica and Greenland

* Added sx_shelf and sy_shelf as ice shelf diagnostics to save the surface slope fields used in the shallow shelf approximation (SSA). This is particularly helpful for determining whether unrealistic velocities are caused by unrealistically steep surface slopes, which can sometimes arise for example, on coarse grid cells that cover both a steep mountainous region and a realively flat ice shelf. Then, the MAX_SURFACE_SLOPE parameter can be tuned to set an upper bound on the SSA surface slope to avoid these steep-slope-induced problematic velocities.

* Fix doxygen errors for ice-sheet process_and_post_scalar_data routine

* FMA fix associated with enforcement of max allowed ice-shelf surface slope

Author: alex-huth