Molecule metadata (e.g. option block in SDFile) behavior in the next version

[mojaie]

• metadata function would be deprecated (too general and easy to conflict with other packages).
• get_prop(mol::MolGraph, key::String) returns the value of the metadata field.
• set_prop! should work similarly.
• Still get_prop(mol::MolGraph, key::Symbol) returns other graph properties (e.g. stereochem) but in many cases these should be automatically generated and should not be modified.
• Molecules built from SMILES should be able to have manually curated metadata fields.
• convenient metadata setter/getters (e.g. mol["compound_id"] = "ABC00001")