Improve header paths in generated C++ code

[bocchino]

For header files tied to an FPP definition, use the location of the definition as the location of the header. Before we were using the current directory (where the analysis is run) in some cases.

• Run check-cpp
• Test with FppTest

Closes #880.

[Kronos3]

Does this need to be updated as well?

fpp/compiler/lib/src/main/scala/codegen/CppWriter/ConstantCppWriter.scala

Lines 21 to 25 in f2dcf6d

	val cppHeaderLines = {
	val path = s.getRelativePath(s"$fileName.hpp")
	val headers = List(path.toString)
	Line.blank :: headers.map(CppWriter.headerLine)
	}

[bocchino]

Does this need to be updated as well?

I don't see a way to update that one, because FppConstantsAc.cpp contains all constant definitions in the model; it's not tied to any one definition or location. We could add a command-line switch to provide more control over the location, but for now it seems OK to leave the current directory as the location.