Don't ask me for a version fitting user supplied macro functions. First
I don't have time to do it (it would be some work, one would also have
to sepcify the initial parameters, etc.).
Second, fitting macros functions is rather slow, nothing that you want
to do for millions of pixels. Already the built-in 'Exponential with
offset' fit of 512x512x10 pixels need almost a minute on my i7 machine
and lets the cooling fans spin up, even though it effectively fits only
one parameter (the other two are calculated directly by linear regression).