Simplify conditional fed codegen

[edwardalee]

This PR is a small step towards improving the C codegen structure. It includes the following:

• Disallow ports in top-level reactors and remove such ports from one test that had them.
• Merge nearly identical methods in CGenerator into one consolidated method.
• Homogenize the way that the generator determines which components are included in a federate.
• A few other small fixes.

[Soroosh129]

It looks like a few validation tests have ports in the main reactor.

[Soroosh129]

This PR makes the CGenerater code a lot cleaner!
I think we should perhaps wait for @housengw to see if these changes would interfere with his work in #958.

One note: It looks like I had missed the addition of currentFederate as a class variable in a previous PR. I think relying on currentFederate could make it harder to break the CGenerator into multiple, smaller generators (like the CppGenerator).

[housengw]

the code changes that overlap with #958 are minor. Is there a reason behind using currentFederate instead of passing a federate parameter to the code generating functions?

[edwardalee]

the code changes that overlap with #958 are minor. Is there a reason behind using currentFederate instead of passing a federate parameter to the code generating functions?

Every function needs it. We could pass it as a parameter, but it gets pretty tedious.

[housengw]

this PR looks good. We just need to keep track of where currentFederate can be modified..