E(B-V) average over field, not just at boresight

[dstndstn]

For obsbot, we’d want a median E(B-V) over the field rather
than just at the boresite.

Specifically, the tiles file for DECam takes the median of extinction
values spaced every 0.1 deg that fall within 1.0 deg of the boresite.
To avoid spherical trig problems, I placed these positions around
a pole, and then rotate the pole to the boresite.