Tan/Sign: need forward operator returning union of intervals

[gchabert]

To make Forward/Backward optimal for elementary equations such as:
y=tan(x)
or
y=sign(x)
we need a forward operator for "tan" an "sign" to return an union of intervals (like we already have for the division with div2 and div2_inter). So we need to develop something like tan2_inter(x,y,out2) and sign2_inter(x,y,out2).