#117 Limit number of major iterations

[André Offringa]

In some cases WSClean gets stuck in doing major iterations: inversion, prediction, a few clean iterations and start again, after which the peak flux has not decreased. Because neither the peak flux goes down nor the nr. of minor iterations goes up (much), this can continue for a long time. While ticket #31 (check for diverging clean) might solve a bit of the problem, in these cases clean is actually not really diverging. A limit on the number of major iterations will make sure that at least WSClean ends in a reasonable time. Reported by N. Hurley-Walker.

[André Offringa]

Option -nmiter was added to limit the nr of major iterations. I might set an upper limit of 20 by default, will think about this.