Replace missing solutions by average gain

Created by: aroffringa

During solution propagation, instead of replacing missing (nan) solutions by unity, this commit will replace missing solutions by the average absolute gain. I decided to use average absolute instead of normal (complex) average, because otherwise this might result in near-zero result when the phase changes a lot over stations. This is related to #81 (closed).

I've tested it on one of Francesco's sets, showing that i) it is correctly implemented and ii) it solves #81 (closed) in that particular case, but it's not yet clear whether #81 (closed) is solved in its entirely.