BUG: fixes issue #4322

Adds support for the thousands character in csv parser for floats.

Updated docs to reflect bug fix.
TST: Removed double import in test io.tests.parser_tests.
ENH: avoid branch in thousands check. Thanks to @cancan101
Merge pull request #4598 from guyrt/csv-import-arg-conflict

csv_import: Thousands separator works in floating point numbers
TST: test for GH4633, bool(obj) behavior

BUG: GH4633, rever to GH1073, whereby __nonzero__ always raises for all NDFrame objects