Area-preserving bilinear interpolation for Healpix maps

[arahlin]

The weights computed for bilinear interpolation are computed in (theta, phi) coordinates, such that the effective sample location is shifted north or south compared to its actual location, with a shift amplitude that scales like 1/nside and is largest near the poles. This PR changes the GetInterpPixelsWeights method to compute weights in (z, phi) coordinates, so that the weights are area-preserving and calculated in the coordinates in which the Healpix pixel boundaries are defined.

Note that this means that HealpixSkyMap.get_interp_values() no longer produces the same results as healpy.get_interp_val().

Thank you to Eric Hivon for identifying the issue.

[hivon]

I see one problem with the new code, not affecting SPT (since it is for the southernmost ring): line 364 should be
double wz = (z - z1) / ( -1.0 - z1);

[hivon]

Actually, there is similar problem at the north pole of Healpix: line 353 should read
double wz = (z - 1.0)/(z2 - 1.0)

[yomori]

Tried mock-observing and verified that the code works.