Merge pull request #4675 from jreback/numeric

CLN/API: Refactor of _get_numeric_data/_get_bool_data to core/generic.py, allowing Series/Panel functionaility
TST: test for GH4633, bool(obj) behavior

BUG: GH4633, rever to GH1073, whereby __nonzero__ always raises for all NDFrame objects
DOC: docs to add detail on gotchas for true value testing