Specifying coordinate systems in the interface.ccl

Coordinate systems associated with grid variable groups can be specified in the group's tags table, using the key COORDSYSTEM. Below is a grid array which could represent a vector field on a 2-sphere.

CCTK_REAL SphericalVectorField TYPE=ARRAY DIM=2 TAGS='COORDSYSTEM="sphere2d" TENSORTYPE="vector"'
{
  field_theta, field_phi
}

Even though another thorn has set a default for all 2D grid variables to something else, Coord_GroupSystem() will always return the handle for sphere2d when called on this group.