move ndarray-returning functions to EA mixin classes
Orthogonal to other EAMixin PRs.

The methods this moves are unchanged.  Docstrings changed e.g. "DatetimeIndex" to "Datetime Array/Index".  I'm open to suggestions for better wording.

The error message in `PeriodArrayMixin._maybe_convert_timedelta` is changed to use `type(self).__name__` instead of hardcoding `PeriodIndex`.  Ditto the warning issued by `PeriodArrayMixin.freq.setter`.
Hello @jbrockmendel! Thanks for updating the PR.

















Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on July 04, 2018 at 17:06 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21722?src=pr&el=h1) Report
> Merging [#21722](https://codecov.io/gh/pandas-dev/pandas/pull/21722?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2b136054db3b8f4c8d1aa33160c4bbe3b8ce940e?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `87.09%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21722/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21722?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21722      +/-   ##
==========================================
+ Coverage   91.92%   91.93%   +<.01%     
==========================================
  Files         158      158              
  Lines       49702    49716      +14     
==========================================
+ Hits        45690    45704      +14     
  Misses       4012     4012
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <87.09%> (ø)` | :arrow_up: |
| #single | `42.01% <38.7%> (+0.01%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21722?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/21722/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.48% <ø> (-0.01%)` | :arrow_down: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/21722/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `100% <100%> (ø)` | :arrow_up: |
| [pandas/core/arrays/timedelta.py](https://codecov.io/gh/pandas-dev/pandas/pull/21722/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3RpbWVkZWx0YS5weQ==) | `100% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/21722/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `93.35% <100%> (+0.81%)` | :arrow_up: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/21722/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `96.98% <100%> (+0.11%)` | :arrow_up: |
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/21722/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `91.05% <100%> (-0.04%)` | :arrow_down: |
| [pandas/core/arrays/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/21722/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3BlcmlvZC5weQ==) | `86.53% <80.55%> (-13.47%)` | :arrow_down: |
| [pandas/core/arrays/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/21722/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lbGlrZS5weQ==) | `94.38% <94.11%> (-0.14%)` | :arrow_down: |
| [pandas/core/reshape/tile.py](https://codecov.io/gh/pandas-dev/pandas/pull/21722/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS90aWxlLnB5) | `94.73% <0%> (ø)` | :arrow_up: |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/21722/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `83.57% <0%> (+0.03%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21722?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21722?src=pr&el=footer). Last update [2b13605...1a3fdef](https://codecov.io/gh/pandas-dev/pandas/pull/21722?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. ping on green. ping thanks!