Renamed 'input' to 'total_input' to clarify that it is sum of bmelt and (possible)
external (en/supraglacial) input to the bed.  There are two versions of the method
that fills this field, one which assumes that the only input is bmelt, and that
it is independent of time during the time step.  The other version
get_input_rate_time_varying() reads the additional time-dependent input from
IceModelVec2T *inputtobed.  The code in various update() methods avoid calling
get_input_rate_time_varying(), which is more expensive, unless inputtobed is
actually present.
Various edits without change to functionality or major addition of documention.  Except I am starting to revise subsection 5.14 to reflect recent additions of hydrology.