Mistake in drawfile_options structure

[Alexander Thoukydides]

The "grid_spacing" field in "struct drawfile_option" is defined as being a
"float", but PRM4-467 shows this an a eight byte value. Looking at files
saved by Draw, I'd say that the PRM is right.

Hence, this field should be changed to a "double".

Cheers,

Alex
-- 
--  Alexander Thoukydides  Email: mailto:alex at thouky.co.uk
--  Cambridge, England       WWW: http://www.thouky.co.uk/
--  A mouse is an elephant built by the Japanese.