Implement rewrite as Sum helper for the Product
Implement numerical evaluation of products (closes #7651)
Add check for finite limits in evalf_prod: use doit() for this case
Use evalf_sum to evaluate Sum after rewritting the Product

In case evalf_sum can't do the Sum - it should raise NotImplementedError.
Then, evalf can return the original expr for the Product.
Add missing import
Revert last two commits
Merge pull request #7652 from skirpichev/evalf_prod

Implement numerical evaluation of products