Documentation says that Fitzpatrick 1999 model is default

Updated the inline documentation to note that Fitzpatrick 1999 model is
the default, as this is otherwise not visible to the user.
Added documentation for Draine dust models
Added wd01 model

wd01 model is only defined at r_v = 3.1, 4.0 and 5.5, although we
interpolate to all wavelengths. Added the datafiles describing this
model. Currently normalize extinction to be 1 at 5495 angstroms, in
keeping with CCM89.