Improve dataset definition API

Using expected format of output from Geant4's main CMake system,
set up dataset variables as per current Geant4 trunk.

Export dataset information as a description list, which is a CMake
list of tuples, one tuple per dataset. Each tuple contains entries for
the dataset name, environment variable and configured path. Parse
description list to set the following make variables:

- G4DATASETS : gmake list of dataset names
- G4DATASET_<NAME>_ENVVAR : Environment variable used by Geant4 to
  locate dataset <NAME>.
- G4DATASET_<NAME>_PATH : Path to dataset <NAME> configured by Geant4.
  Does not guarantee path exists or is readable.

Export a phony description list from main CMake script, outputting
all variables in geant4_help_datasets target.

NB: Processing does not handle spaces in dataset paths.