Add --executable option to process command

[goodmami]

ACEProcess subclasses can be instantiated with the executable parameter to specify an ACE location if it is not on PATH, but this is not available to the delphin process command, meaning that users of the command are restricted to using ACE on PATH. To fix this deficiency, add an executable parameter (str or pathlib.Path) to the delphin.commands.process() function and add the --executable option to delphin process.