Abstract types should be skipped from serialization

[EMaksymenko]

After implementation of derivationBySubsumption option all abstract types, which were replaced by its children types, remains empty (without any attributes) in the result proto files and they never used in any result messages anymore.

There should be an option like skipEmptyTypeInheritance, but for abstract types replaced by derivation, e.g. skipAbstractTypes

Or may be remove abstract types by default when derivationBySubsumption is enabled, because anyway messages without fields are useless.

[EMaksymenko]

I think even abstract types which have own fields (not only empty one) should also be skipped, because they also should be replaced by derivationBySubsumption option and never used directly for data serialization.

That's why I propose to skip any type which have attribute abstract = "true" regardless of its content.