Fix include issue with Branch.hpp (#57)

[alexander-novo]

Fixes #57

Also brought some adjacent includes in line with style guidelines

[alexander-novo]

This should fix the mis-compilation being experienced in the CI of #32

[pelesh]

Power flow branch header Branch.hpp is missing forward declaration to BranchData class. Adding the forward declaration should fix this issue.

The currently proposed solution introduces additional build dependencies that would be good to avoid.

[alexander-novo]

introduces additional build dependencies that would be good to avoid

@pelesh for future reference can you explain what you mean by this?

[pelesh]

introduces additional build dependencies that would be good to avoid

@pelesh for future reference can you explain what you mean by this?

In header files use forward declarations instead of including other headers whenever possible. That way, we have better control over what is included in each compilation unit.

[alexander-novo]

Changed to adding the forward declaration instead of including the definition. In the process I realized the phasor dynamics implementation already had this forward declaration, so we should be all good there.

@pelesh this forward declaration philosophy might be a good thing to put in the developer guidelines, since it seems like an easy thing to catch people unaware.

[pelesh]

Good to merge, thanks!