Added the option to set a close_to_point tolerance in PointLocatorTree and MeshFunction, as a fallback option.

This is helpful when we have numerical tolerance issues which can lead to points being outside a mesh within
some tolerance. Note that we use a linear search with close_to_point, so it's slow, but at least it's a robust
backup.
Merge pull request #309 from dknez/master

Added the option to set a close_to_point tolerance in PointLocatorTree