Writes a file which can subsequently be read into MONGO. It is
possible to make MONGO leave gaps in plots, corresponding to datum
points with a notifiable `bad' y value; the MONGOWR command inserts a
`bad' point with y value `badval' (default 0) into breaks in the
data set to facilitate use of this option.
Most of the more important MONGO functions are reproduced in DIPSO -
you shouldn't have to use this command!