moved base classes into categorized modules (#126)

ale/base/__init__.py

ale/base/base.py

ale/base/data_isis.py

ale/base/data_naif.py

ale/base/label_isis.py

ale/base/label_pds3.py

ale/base/type_distortion.py

ale/base/type_sensor.py

ale/drivers/cassini_drivers.py

ale/drivers/dawn_drivers.py

ale/drivers/kaguya_drivers.py

ale/drivers/lro_drivers.py

ale/drivers/messenger_drivers.py

ale/drivers/mro_drivers.py

tests/pytests/test_dawn_driver.py

tests/pytests/test_isis_spice_drivers.py

tests/pytests/test_lro_drivers.py

tests/pytests/test_mdis_driver.py

tests/pytests/test_mro_drivers.py