Double check Ref Pixel calculations

[Joshuaalbert]

Ensure that l0, m0 calculations are correct.

We have that w-gridder uses:

# 0-indexed
lvec[i] = (-0.5 * N + i) * dl + l0

hence lvec[N//2] = (-0.5 * N + N // 2) * dl + l0 = l0 (because N always even). But in FITS I think pixels are 1-indexed, so in pixel frame the ref pixel CRPIX = N // 2 + 1.

Should verify empirically though.