The presence of this variable is interesting, given we have a ```spherical``` variable defined just above.
The two names could easily cause confusion; they both initially seem to mean "x is round".
Of course, we know "spherical" refers to the space in which the coord sits, and "circular" refers to the extent of it's values (and if it's spherical). Something "underlying" feels wrong but I can't quite put my finger on it yet.

Perhaps "spherical" should be a property of a Coord. I won't suggest it's renamed to "can_be_circular" but "spherical" seems ab inappropriate term for a 1D coordinate.