refactored type annotation to be able to resuse union types

[tylerflex]

• moves TYPE_TAG_STR to types.py
• introduces annotate_type(UnionType) function to wrap the adding of discriminator to Union[...] type.
• puts annotations directly in Field() definitions with Tuple[Union[], ...] fields
• adds discriminator to pd.Fiedl() for Union[] fields.

[shashwat-sh]

Looks good to me. I'm having unrelated problems with compiling the solver after the recent CMake changes, so I wasn't able to run backend tests. But since the frontend test_IO is passing, there probably shouldn't be any issues.