Throw an error on non-convergence

[rbooth200]

With alpha~1 frank can need more than the default max_iter=2000 to converge. We'd never really worried about this because the best fit is typically hardly changing long before we accept convergence so for practical uses it had been fine to print a warning and rumble on.

I've now come across a case where it is not fine to rumble on, in the work on the debris disc fitter paper with Seba Marino. We're using the posterior probability of the best fit and this is not reliable if we haven't reached convergence.

I'm now preposing that we change the default behaviour to raise an error if the code has not converged in max_iter, while giving users an option to skip it, just printing a warning if convergence hasn't been reached. This would be the 'pythonic' solution. Whatever we decide on, we need a change because currently the code doesn't even print a warning if logging is not enabled.

I'm happy to implement a solution when agreed on.