Allowing users to specify which multimaterial mean to use in the bulk_viscosity algorithm.

This has been implemented more generally so that all multimaterial averaging routines are capable of it regardless of rank but
nothing other than bulk_viscosity actually exposes the functionality.

A (very) short test has been added.
Make fluidity build with petsc 3.5

PETSC_DEFAULT_DOUBLE_PRECISION changed to PETSC_DEFAULT_REAL
KSP/PCSetOperators no longer have a MatStructure flag
As before, previous versions (3.1-3.4) are supported through macro hacks in include/petsc_legacy.h

Fixes #17
Remove depracated extended source stuff from configure.
Revert "Remove depracated extended source stuff from configure."

This reverts commit 9a85c0d7638f524e1f31e7853bc54aa34c63d41f.
DMPlex: Adding option and test for DMPlex-ExodusII mesh reader
DMPlex: Build and distribute a DMPlex from an ExodusII mesh file