Add `fastai` support

[aguschin]

If we add this support in May, @alex000kim could use it to showcase MLEM in his tutorial.

[aguschin]

@alex000kim could you please share a link to your project so we could check this?

[alex000kim]

Sure, here it is: https://github.com/iterative/magnetic-tiles-defect

[aguschin]

Mike is doing this in #268