Implement C-API accessor to evaluator topology

This way external application might check whether it need to
re-create evaluator from scratch or it might re-use existing
one without storing extra data from it's side.

TODO: Tags are still not accessible via C-API, it's marked as
to be solved later when it becomes more clear what is the best
way to expose them and when there'll be a real application to
test the tags.
Add getter of topology object to evaluator C-API

Useful for cases when some parts of the pipeline needs to know the
topology object. Simply helps saving memory so topology from the
evaluatgor can be re-used for stuff like HbrMesh creation.
Code cleanup: no need to use struct keyword in C++ code

It's only required for anonymous structures in C.

Also made a bit of whitespace cleanup.
Merge pull request #299 from Nazg-Gul/dev

Implement C-API accessor to evaluator topology