Plot ISMIP6 thermal forcing time series for Amery/Lambert UQ paper.

Add option to plot SMB across a region or the entire domain.

Update linestyles based on ESM, save a pdf version, use lowercase
axis labels, etc.

Support plotting average over depth range, for example using '-d -200,-500'
in the command.

Add option to plot over whole shelf depth range, using the
--shelf_range flag. This may not be very accurate because it will
sample deep temperatures far from the grounding line, where they
are unlikely to ever be felt by the ice shelf during a simulation,
but it is much faster than interpolating to a specific depth or
to the ice-shelf base or seafloor.

Co-authored-by: Matt Hoffman <wyeast@gmail.com>