Fix for the internal structural object atomic coordinate assembly function.
This is the pipe_control.structure.main.assemble_structural_coordinates() function. The case of no
sequence alignment being required as only models are being handled is now functional. The strings
and gaps data structures passed into the lib.sequence_alignment.msa.msa_residue_skipping() function
for generating the residue skipping data structure are now set to the one letter codes and an empty
structure of zeros respectively.
