Adding time limit to levenberg_marquardt.jl

[Gkreindler]

This small edit allows levenberg_marquardt() to exit if a specified time limit is exceeded.

This is my first ever pull request, so any tips or pointers to best practices are welcome!

[Gkreindler]

Hello, any change someone might be able to review this? I think it is very simple, and I would like to use this feature in a package that I am working on.
Thank you in advance!

[pkofod]

This is my first ever pull request, so any tips or pointers to best practices are welcome!

I'm sorry for not reviewing, I appreciate the effort! I changed your initial value from -1 to NaN. I don't think it makes sense to restrict it to integers as time() returns a Float64 value. By setting it to NaN we can be sure that time() - t0 < time_limit is never true, because no number is less (or larger) than NaN.

The other comment I have is that typically you'd be required to add a test. Sometimes it's a bit hard to come up with a test. In this case you could maybe use a model that would otherwise converge fine, but that has sleep(10) in it and set the time limit to 1 such that you know it will be violated on the very first iteration. Then, you can test that the number of iterations was indeed 1, and then afterwards test the same model without the sleep and verify that it did indeed converge fine without a time limit.