Added a "covered_by" algorithm (= within or on border)
Revised point-in-box, box-in-box, now with strategies
Therefore, revised within strategy.
Also adapted concept for within, and refactored getting parameters into parameter_type_of (also used by distance concept)
To avoid specifying default strategies again and again for all spherical coordinate systems, used the spherical_tag as the parent for spherical_polar_tag, spherical_equatorial_tag, geographic_tag