Test the O'Donnell 1998 extinction law against SFD98

Added a test of the O'Donnell extinction law against the values tabulated in Schlegel, Finkbeiner and Davis (1998). Currently the test fails because the values do not always agree within the reported significant figures in SFD98 (0.001 mags). However, the values do agree to within 0.005 magnitudes.

![sfd_od98_test](https://f.cloud.github.com/assets/3639698/1552584/46ea0372-4e26-11e3-88dc-3b1b901b3537.png)