Add getPosition and getTangentPlaneOffset methods from Coord
to SpherePoint. Simplify the former by requiring the units argument
and not treating hours specially. Fix documentation errors in the
latter (it claimed the offset was to "this" but it's from "this").

Some exising users of Coord rely on a default constructor,
so add one to SpherePoint.

and related classes and functions, including CoordSystem, makeCoord, makeCoordEnum,
and assertCoordsAlmostEqual

in test_warpExposure.py

The simple.cc pybind11 wrapper imports lsst.afw.coord.
It should not be necessary, but removing it causes problems
so I made a note with a reference to a possibly relevant RFC.