Added test for O'Donnell 1998 model against SFD paper

Tests the O'Donnell 1998 extinction curve against the values tabulated
in Schlegel, Finkbeiner and Davis (1998). Currently, results agree to
0.01 magnitudes, but SFD gives values to 0.001, and many points fail at
this precision.
Removed test plot commands

Removed commands to plot the difference against wavelength.
Updated name of polynomial Legendre model
Marked SFD98 test as xfail
Merge pull request #43 from jfoster17/master

Test the O'Donnell 1998 extinction law against SFD98