Allow the user to limit point locator searches to subsets of the mesh.

We have an application where a requested point lives on the interface between subdomains, and it is desirable to restrict which subdomain the returned element belongs to. This commit implements this feature, with backwards compatibility through the API using default function arguments.
Merge pull request #365 from libMesh/restricted_point_locator

Allow the user to limit point locator searches to subsets of the mesh.