Expand to_smiles API

[jthorton]

Is your feature request related to a problem? Please describe.
In order to produce more Cmiles identifiers, we need to expand the to_smiles API to allow for extra arguments which define the flavour of the smiles returned. Such as isomeric, explicit_hydrogen and mapped.

These options would allow the creation of the following variants

canonical_smiles = mol.to_smiles(isomeric=False, explicit_hydrogen=False, mapped=False)

canonical_isomeric_smiles = mol.to_smiles(isomeric=True, explicit_hydrogen=False, mapped=False)

canonical_explicit_hydrogen_smiles = mol.to_smiles(isomeric=False, explicit_hydrogen=True, mapped=False)

# default 
canonical_isomeric_explicit_hydrogen_smiles = mol.to_smiles(isomeric=True, explicit_hydrogen=True, mapped=False)

canonical_isomeric_explicit_hydrogen_mapped_smiles  = mol.to_smiles(isomeric=True, explicit_hydrogen=True, mapped=True)

are there any other options which we should include at this point?