Some additional models based on astropy.modeling.
These are for my own use, but made available in case they are useful
for others. Ultimately, I hope to contribute these for inclusion in astropy.modeling.
Currently the only model implemented is a simple asymmetric version of the 2D Sérsic profile.
It may be better for this to be implemented as a wrapper that adds asymmetry to any 2D profile.
I also plan to add a 2D "higher-order Sérsic" profile suggested by René Andrae, Peter Melchior and Knud Jahnke (2011) (see appendix A2). The regular Sérsic profile is characterised by a single 'index' (n), which determines the profile shape at both large and small radii. The higher-order version extends the regular Sérsic profile, using Taylor expansion arguments, such that the behaviours at small and large radii are decoupled.