Sets the search path for external modules. The PATHi should
be pathnames of directories containing, for each module, the
module's executable file and a .geomview-<modulename> file
which contains an (emodule-define ...) command for that
module. This command implicitly calls (rehash-emodule-path)
to rebuild the application brower from the new path setting.
The special directory name + is replaced by the existing path,
so e.g. (set-emodule-path (mydir +)) prepends mydir to the path.