Add function to check if a pdf depends on a given variable.
Add functions to change the limits of a pdf for a given variable and renormalize accordingly.

ALERT!: It is still very experimental, since the resulting pdf is scaled by a factor to do the
        renormalization, and it's not yet clear how to operate with pdfs that have been scaled.
Make sure the pdf always evaluates to a non-negative value, whatever the
        values of the Function objects are. Default to zero if the product
        of functions is negative.