dht cleanup
`plot.ds` -- now uses the `which` and `pages` arguments, `which`
indicates which plots (can be a single number of vecctor) you want and
`pages` indicates how many pages you'd like them over. It uses R's
`devAskNewPage()` to do cycling, which probably won't play nice with Distance, but I guess you're using `which` one at a time anyway.
Plotting functions can now use a pages argument. This works exactly as
in mgcv. length(which) plots will be split over pages number of pages.

New function plot.layout does the work.

Most code stolen from plot.gam in mgcv. Thanks Simon!
Merge branch 'plotwhich'

Conflicts:
	mrds/man/plot.io.Rd
	mrds/man/plot.io.fi.Rd
	mrds/man/plot.rem.Rd
	mrds/man/plot.rem.fi.Rd

(documentation conflicts)
documentation update
clean up ds.function, that isn't really even used, so good use of time
apex.gamma now just takes a ddfobj as argument
Merge branch 'dht-cleanup'
minor dht cleanup