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
DOC: indexing.rst edits
Merge pull request #4598 from guyrt/csv-import-arg-conflict

csv_import: Thousands separator works in floating point numbers
BUG: allow as_index=False for groupby apply
TST: test with all types of indices