Implement rewrite as Piecewise for Abs, sign and Heaviside (issue 3478)
Cleanup _eval_rewrite_as_Heaviside() method for Abs
Merge pull request #1956 from skirpichev/3478-rewrite-to-Piecewise

Implement rewrite as Piecewise for Abs, sign and Heaviside (issue 3478)