Add frequency + fallback value selector

[cprudhom]

There's no doubt that BIVS is very useful. However, it is also time-consuming and it is not always appropriate to use it. I propose, in this PR, to make it possible to indicate an alternative frequency and strategy.

[jgFages]

I agree that BIVS is costly. However, I would prefer the condition to be a generic function : IntVar -> Boolean. One implementation could be to be based on the number of backtracks, another could be based on the variable's name or domain. It could also be random...

[cprudhom]

Ok, I can change to a more functional behavior even if sometimes when there's too much freedom you don't know what to choose :)

[cprudhom]

I propose to define the default application of BIVS each time, i.e. v -> true in order to keep the performance tests unchanged.