relax SVN archive

Fix for the unit test of the generic_fns.structure.pdb_write.helix() function.

The init_seq_id argument is now called init_seq_num to match the PDB notation.

Fixes for some unit tests of the internal structural object for the changes to the atom_loop().

Single elements are now returned as single elements rather than a tuple of length 1.

Docstring editing of the ScientificPython structural object atom_loop() method.

Added the index_flag argument to all structural API atom_loop() methods.

Created 13 unit tests of the Internal._trim_helix() internal structural object method.

SHEET PDB records are now read, stored, and written out by the internal structural object.

This affects the structure.read_pdb and structure.write_pdb user functions. The sheet is stored as
a metadata type object - its elements do not correspond to the atoms in the structural object.

HELIX PDB records are now read, stored, and written out by the internal structural object.

This affects the structure.read_pdb and structure.write_pdb user functions. The helix is stored as
a metadata type object - its elements do not correspond to the atoms in the structural object.

Fix for the internal structural object _pdb_chain_id_to_mol_index() method.

This was accidentally set to be a method of the MolContainer class and not the internal object.

Created the internal structural object _pdb_chain_id_to_mol_index() method.

This will be used to convert PDB chain IDs, which are used to indicate different molecules in the
PDB, into molecule indices for the internal structural object.

Trailing whitespace removal.