Allow provider-specific data types

[merkys]

This PR attempts to generalize #436 by allowing to define provider-specific data types. The following conditions for such data types apply:

• Their values have to be expressed as string literals both in responses and in filters;
• They have to be named using provider-specific prefixes;
• Properties can only be compared with values of the same type (as per specification up til now).

The latter condition might be too strict for some uses, but since casting is unclear at the moment, let us leave such extensions for future.

Merging this PR will allow to reassign #392 and #436 (SMILES) to _cheminfo_ namespace where it can become a property with special set of rules for comparison.