time array tests
make plots solely from cache file

use argument --cache_test on plots.py for demo
front relative time aray
more verbose cli help
include fft in power plot
increase width of fron relative window
plot real and imag fft
allow cli spec of plots to make

default is plot everything
make plot executable
allow both interactive and cli usage of plot
instance variable front offset
more verbose doc
hovmoller plot
compute gradient fields
only compute time of front passage once
single default plots list
correct logic for caching
mean vorticity plot
plot mean shear
compute front velocity