Fixes for the pipe_control.structure.main.assemble_structural_coordinates() function.
The function will now raise a RelaxError if no structural data matching the atom ID can be found.
The problem was uncovered by the Structure.test_atomic_fluctuations_no_match system test. The fix
affects the structure.atomic_fluctuations, structure.displacement, structure.find_pivot,
structure.rmsd, structure.superimpose, and structure.web_of_motion user functions.
