Change default cmap

The default map for the plot.imshow operations is now ‘pink’, not
‘jet’. The rationale for this was that it will give LL a distinctive
visual language distinct from generic-matplotlib, but also - and more
importantly! - this is colorblind friendly.
Tweaks to stream_power for runoff_rate var

Made it clearer that runoff_rate variations can now be accommodated in
stream_power by feeding it arrays of discharges produced by
route_flow_dn under variable runoff.

Note this should work for spatially distributed runoff as well as
time-varying!

Added a couple of assert checks to make sure the user knows what they
are doing if feeding discharges or widths directly.