Consistent and intuitive handling of relative paths

[axch]

The natural interpretation of relative file names in the keyword arguments of driver.simulate is relative to the working directory; whereas the natural interpretation of relative paths in the aronnax.conf file is relative to the location of that file, especially for inputs. We should make, implement, and document a conscious decision about relative path handling during configuration merge and subsequent interpretation; but perhaps that decision can wait until we have more experience using the new Aronnax driver.

[edoddridge]

i've been mulling this over and haven't come up with a definitive answer. Requiring that driver.simulate is called in a directory with an aronnax.conf file somewhat reduces the difficulty of this. In that situation both sets of relative paths are equivalent.