Efficient extinction computation

[RoyWilliams]

The dustmap file is opened and put into memory every time an extinction is computed, then torn down after, resulting in great inefficiency. This cane be made much more efficient.

[gpfrancis]

I did a quick test to compare time in initialisation vs computation:

Case 1:

for i in range(n):
  ra = random.uniform(1,360)
  decl = random.uniform(-89,89)
  sfd = SFDQuery()
  c = SkyCoord(ra, decl, unit="deg", frame='icrs')
  ebv = float(sfd(c))

Case 2:

sfd = SFDQuery()
for i in range(n):
  ra = random.uniform(1,360)
  decl = random.uniform(-89,89)
  c = SkyCoord(ra, decl, unit="deg", frame='icrs')
  ebv = float(sfd(c))

Clearly there is a gain to be had here, but I don't think it's more than a factor of 2.