[Therion] Hiding small objects at large scales

[Bruce]

The hide small rocks layout really works.  Quite nice.

Thomas Holders scalebar1 is also very nice, although I can only get it to
work at 1:1000.  Get error cp-02 for larger and smaller scales....
Not quite sure why, Mostly I'm ensuring the 'as plotted' length of the
scalebar is 100mm so it shouldn't be a formatting problem.

Still haven't cracked the syntax for label size.
Are you going to make me lookup the metapost references :)

>if and(size <> xs, size <> s)
> [standard label metapost code]
>fi;

Bruce