[GenABEL-dev] "imputations" section of the tutorial

[Yurii Aulchenko]

Just to keep track of this action point: we definitely need to update
the "Genetic data imputations" chapter -- it is outdated to the point
it is harmful, cause people use that in courses (and some times we
even would not know, so can not tell 'beware!'). We can base the
revision on ENGAGE Barcelona workshop material and ProbABEL paper.

Yurii