Categorical: .codes should be immutable

ERR: codes modification raises ValueError always

Categorical: use Categorical.from_codes() in a few places

Categorical: Fix assigning a Categorical to an existing string column
CLN: CategoricalDtype repr now yields category
DISPLAY: show dtype when displaying Categorical series (for consistency)
BUG: fix groupby with multiple non-compressed categoricals
Categorical: minor doc cleanups