Skeleton for elliptic integrals
major rewrite of pull #1408:

    * change names
    * docstrings
    * some documentation
    * mpmath translations
    * eval(), fdiff(), some rewrite methods
Added tests for elliptic integrals
Some support for latex printing of elliptic integrals
Add tests for elliptic integrals in test_args.py
Add complete elliptic integrals to the table for hyperexpand()
Merge pull request #1977 from skirpichev/2063-elliptic_integrals

elliptic integrals (issue 2063)