Relax check in test on float equality.

Check was on number of places, and the rounding used could trigger a failure for tiny changes in output.
Small doc fixes.
tiny fix to cast a 0 to std::size_t
Merge branch 'garth/fix-64-bit-issues' of ssh://bitbucket.org/fenics-project/dolfin into garth/fix-64-bit-issues
Merge branch 'garth/fix-64-bit-issues' into next