Created the Frame_order.test_generate_rotor2_distribution system test.

This is to test the Frame Order distribution generating base script, used for creating the synthetic
Frame Order test data, and to demonstrate a failure in handling back-calculated RDC data.

To implement this, the test_suite/shared_data/frame_order/cam/ path has been converted into a Python
package (with the addition of the __init__.py files).  The base data generation script
test_suite/shared_data/frame_order/cam/generate_base.py has also been modified to use the absolute
path for the data files and its run() method now accepts the save_path argument to allow the files
to be saved into a temporary directory.
Fix for bug #21499 (https://gna.org/bugs/?21499), the failure of the rdc.write user function.

The rdc.write user function fails for back-calculated RDC data.  The fix was to handle the missing
interatom.rdc_data_types variable.
Fixes for the Frame_order.test_generate_rotor2_distribution system test.

The test_suite/shared_data/frame_order/cam/generate_base.py script now saves the program state files
into the self.save_path directory, preventing the system test from attempting to save files into the
relax test suite directories.