Correct Pow.as_real_imag() for im=0 (see issue 3474)

    This was fixed before for issue 3162, see PR #1161
Merge pull request #2026 from skirpichev/3474-fix-as_real_imag

Correct Pow.as_real_imag() for im=0 (see issue 3474)
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