wsclean needs a three-way mf-weighting option

[o-smirnov]

The current cab has a boolean nomfsweighting option to force-disable MF weights. This is insufficient, because according to https://sourceforge.net/p/wsclean/wiki/MFSWeighting/, there is also an opposite force-enable option. The reason for needing both is that the default MF weighting choice depends on join-channels. At present there's no way to force-enable at all in the cab.

Ideally we need a three-way True/False/None option in the cab spec, with None having the usual meaning of default. So perhaps a solution within the framework of #637 would be easiest, and we muddle along until then.