Progress bar

[Datseris]

Hi,
I'm using curve_fit extensively in my work. Depending on the fit function I use, there is a pretty large variability in runtime. That's normal. But for some models I have to wait seconds, for some others, tens of minutes. It would be really nice if we can add a progress bar in the main evaluation loop.

I'm happy to contribute this feature, as I've used progress meters a lot in other packages using ProgressMeters.jl. However, I'd like your advice on where is the main computation loop that a progress bar should be wrapped over.

[pkofod]

Well, depends.. The progress of the gradient norm towards the tolerance? Number of iterations out of max?