Make all paths in the experiment relative except for the base path

[rklasen]

Right now there is a lot of duplication in an experiment config. That is not necessary, and more importantly the use may want to change the base path manually and still have a valid experiment config.

Right now, if the base path changes, all occurrences must be changed (not a huge deal, that's what ctrl-R is for). But it would be cleaner if the base path would be there only once.