Avoid polluting public interface with parmetis.h

[Prof Garth Wells]

To clean up the build and linking process, we should avoid exposing dependencies unnecessarily in DOLFIN header files.

The function

https://bitbucket.org/fenics-project/dolfin/src/4fdebe00790a2b335fe5003aa9bd507780cf0186/dolfin/graph/ParMETIS.h?at=master&fileviewer=file-view-default#ParMETIS.h-69

should be moved to the cpp file, and the ParMETIS include should also be moved into the implementation file.

[Prof Garth Wells]

Avoid exposing ParMETIS in public header file. Fixes #782.

→ <<cset 67e7b97da881>>