Fix/disjoint

PR fixing Areal/Areal version by replacing `point_on_surface()` with `point_on_border()` and `within()` with `covered_by()` for backward compatibility (https://svn.boost.org/trac/boost/ticket/8310).

Related to https://github.com/boostorg/geometry/pull/159 and https://github.com/boostorg/geometry/pull/162.

Why not combine the two if statements into one?