Pr/3923 

a suggested redo of gh-3923: this adds a `check_finite` flag to `curve_fit`, modelled after a similar flag in `linalg`. IMO having this flag is a reasonable price to pay for fixing the nonsense of gh-3422, but apparently, there is a range of opinions, see gh-3923 for discussion.

I think it would be nice to fix the original issue, gh-3422, for 0.15.
This PR and gh-3925 provide slightly different ways of fixing it, and I personally would be fine with either of them.