The routine uvopen opens a uv data set and readies it for access.
Here dataname is a string giving the data set's name. Status can
be either 'old' or 'new', depending whether an old data set is
being read, or a new data set is being created. Tno is an integer
handle passed back by the open routine (and is used for all future access
to the data set). The routine uvclose closes the data set.
The routine uvrewind positions a uv file at its beginning, and
allows it to be read again.