Allow Molecule.from_file to read only a subset of molecules/conformers in a file.

[andrrizzi]

For files containing a lot of molecules (e.g. data/molecules/MiniDrugBank.sdf) this should help cutting the time necessary to run the toolkit.

[jchodera]

What would the API look like for extending Molecule.from_file()?

Perhaps

# Read only the first molecule
one_molecule = Molecule.from_file('data/molecules/MiniDrugBank.sdf', molecule_indices=7)
# Read specific molecules
molecules_from_indices = Molecule.from_file('data/molecules/MiniDrugBank.sdf', molecule_indices=[0,3,17])
# Read a range of molecules
molecules_from_range = Molecule.from_file('data/molecules/MiniDrugBank.sdf', molecule_indices=range(10,20))
# Read all molecules
all_molecules = Molecule.from_file('data/molecules/MiniDrugBank.sdf')

[andrrizzi]

That's exactly what I had in mind.

[mattwthompson]

Related #282