Fixes for the read() function to allow the error column to be missing.
Made the error column optional in the loading of RDCs.
Converted the read() function from the relax_data code.
Created 2 new error classes, RelaxNoRDCError and RelaxRDCError.
Created a generic_fns rdc module by copying the relax_data module.
Converted all of the rdc user functions from the relax_data code.
Converted all instances of 'relax_data' to 'rdc'.
Converted all instances of 'ri_label' to 'id'.
Converted the rdc.read() user function from the relax_data.read() code.
Removed some debugging code.