CLN/DOC/TST: categorical fixups (GH7768)

[jreback]

replaces #7768

closes #3678

Categorical([1, np.nan]) would end up with a single 1. float level.
This commit ensures that if values is a list of ints and contains np.nan,
the float conversation does not take place.

Categorical: fix describe with np.nan

Categorical: ensure that one can assign np.nan

Categorical: fix assigning NaN if NaN in levels

API: change default Categorical.from_codes() to ordered=False

In the normal constructor ordered=True is only assumed if the levels
are given or the values are sortable (which is most of the cases), but
in from_codes(...) we can't asssume this so the default should be
False.

Categorical: add some links to Categorical in the other docs

Categorical: use s.values when calling private methods

s.values is the underlying Categorical object, s.cat will be changed
to only expose the API methods/properties.

Categorical: Change series.cat to only expose the API

Categorical: Fix order and na_position

Categorical: Fix comparison of Categoricals and Series|Categorical|np.array

Categorical can only be comapred to another Categorical with the same levels
and the same ordering or to a scalar value.

If the Categorical has no order defined (cat.ordered == False), only equal
(and not equal) are defined.

DOC: use okexcept in docs rather than try: except:

CLN: revised isnull treatement

TST: test for tab completion

[jreback]

@JanSchulz this whole biz seems to work, but seems a bit clunky, let's revisit at some point.

[jreback]

closing in favor of new one from @JanSchulz