add a filter command with spectacular rolloff
change filter so that is sub-samples

If no filter width is given is assumes 4*the sub-sample spacing
Filter adcp data to remove tidal stuff
soften the filter
use datenum to choose date range
restore square hann window

This shoud assure a fast rolloff but has a wide shoulder
change the time windows

longer total record but shrter averaging time
change dz matrix to allow unevenly spaced data
plot filter response if no arguments are given
separate filename into pieces, modify plots
filter Aurelies gridded data, drhodz looks bad:(
test routine to make interactive plots of filtered data
add Pressure and depth
add depth switch to filtered lat/lon
add some actuall plotting comands
create an array NaN where bad data polutes the derivative and zero otherwise
modify help text
tweak sample plot
put filtered TCCham data in struct, move plots to separate file
take mean of an array with nans