interfaceProperties: Add support for re-read of sigma and cAlpha

Note that currently when transportProperties is changed sigma is re-read
from this dictionary but cAlpha is read from fvSolution but cAlpha is
not re-read when fvSolution is changed but transportProperties is not
touched.  To avoid this problem cAlpha would need to be moved to the
transportProperties dictionary which is not ideal or moved out of the
interfaceProperties class back to the top-level solver.

Resolves bug-report http://www.openfoam.org/mantisbt/view.php?id=1657