[phi1, phi2], [theta1, theta2])
return a mesh (see hex_mesh) representing the given section
of the sphere of given RADIUS. IMAX, JMAX, and KMAX are the
number of nodes (cells+1) in the radial, longitude (phi), and
colatitude (theta) directions, respectively. Note that for
a right handed coordinate system, phi1theta2.
interpreted function, defined at i0/hex.i line 432