Fix C++ code generation for ports with string return type

[bocchino]

• Update the code generators for ports and components
  • Incidental: Revise the "FpConfig.hpp" header include to <FpConfig.hpp> to conform to F Prime conventions
  • Regenerate unit tests
• Add unit tests to cover the new behavior

Closes #651.

A small change to F Prime was required. To run the check-cpp tests, use this branch: bocchino/fprime@b6cf5cc.

[Kronos3]

Looks good. I wonder if the if this still works if you return a type with a string nested in another type like a struct or an array?

[bocchino]

It should work because structs and arrays use Fw::ExternalString for string members. Are you suggesting that we add a test case for this kind of return type?

[bocchino]

Note that there are fpp-to-cpp tests for structs and arrays with string members, in the struct and array subdirectories.

[Kronos3]

Ah right, and Fw::ExternalString will have a buffer managed by the port handler in the derived component code so it doesn't concern the base class at all.