Raise exception is nburn if greater than the total number of iterations for emcee

If using the emcee sampler, if you set nburn to be greater than iterations it will remove all your samples as burn-in samples. This patch raises an exception if you try and do this. It also adds to the doc string to make what is done explicit.

Another option would be to change the number of iterations to be iterations+nburn, so that the final number of iterations is still the value that is expected.