Currently `OfType` and `is` only work with the `HiearchicalDiscriminatorConvention`.

This is because with hierarchical discriminators the `_t` value lists all of the classes in the class hierarchy, which makes it possible to write MQL queries for `OfType` and `is`.

To support an `OfType` or `is` query in MQL requires a different query which lists all of the known subtypes in the query itself (rather than relying on them being stored in the `_t` field). This in turn requires that the `ScalarDiscriminatorConvention` implement the new `IScalarDiscriminatorConvention` interface ` and in particular the `GetDiscriminatorsForTypeAndSubTypes`  method.