Created the pipe_control.spectrum.test_spectrum_id() function for checking if a spectrum ID exists.
Renamed pipe_control.spectrum.test_spectrum_id() to check_spectrum_id().

A bug in the function was also removed, and the other code in the module now uses this function.
Created the pipe_control.mol_res_spin.check_mol_res_spin_data() function.

This will check for the existence of molecule, residue and spin data and raise a RelaxError if none
exists.
Simplification of the data checks in the pipe_control.spectrum module.

This is using the new pipe_control.*.check*() functions.