Support iteration strategy for boutiques file

[axlbonnet]

We should add a way to specify in a boutiques file how some inputs combine to form a "dot" iteration strategy.
By default, all inputs are in a "cross".
Using boutiques groups, or input extension, some inputs could be linked in a "dot" group.
In the import interface, this should be visible, and maybe editable ?

[axlbonnet]

done in #480