[Kst] Advance one screen

[Barth Netterfield]

OK: I think we need to differentiate between what we get if we enter f0 and n 
from the command line and from the vector widget, and what we get with 
advance 1 screen.  So:

Entering f0, N which would read past the end with the widgets should:
        f0 is whatever you say.
        The number of points read is either N or the number of points from f0 
to the end of file, whichever is less.

Advancing past the end with advance 1 screen should:
	Read the last N frames.  Q: should it also go into 'read to end mode?'

In all cases, the plot ranges if in autoscale should be based on the data 
which has been read.

cbn