Fix Timestamp rounding
- Closes #21262
- Tests added (thanks  @Safrone [PR](https://github.com/pandas-dev/pandas/pull/21265))

This change-set is to avoid rounding a timestamp when the timestamp is a multiple of the frequency string passed in.

"Values" param passed into round_ns can either be a np array or int. So relevant handling added for both.

FYI I havn't used Cython much before so keen to get peoples thoughts/feedback.

Thanks

Hello @alimcmaster1! Thanks for updating the PR.





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

##### Comment last updated on June 28, 2018 at 20:01 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21507?src=pr&el=h1) Report
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 A few thoughts:

1) With your solution, I think it's better to make `round_ns` only take arrays (therefore wrap scalar inputs as array) to avoid checking the type of the input (and moreover `cdef`ing them) and iterating over values. It appears the solution can still work by computing `values % unit` as a boolean array and operating with that. We do something similar with `tz_localize_to_utc`.

2) Include tests for `DatetimeIndex.round`

3) Looks like there are some linting errors (your spacing seems to be 2 instead of 4 in some places). Thanks @mroeschke for the comments. I've done (2) and (3) added a few additional test cases to the DateTimeIndex ceil/floor and one that clearly shows how round should behave for this bug.

1) Yes this definitely makes sense. Just to clarify are you thinking in 
`_round` we check the type of `value` if its an int we wrap in an np array. Hence we can cdef `round_ns`?

Best,

Alistair

 Rather have `round_ns` (which we can at best `cpdef` since it's imported by a python file) _only_ accept numpy arrays. So since this method is used by timestamps, we would have to wrap it's `self.value` input as a numpy array. The relevant rounding code for `Timestamp`s would turn into:

```
def _round(self, freq, rounder):
    if self.tz is not None:
        value = self.tz_localize(None).value
    else:
        value = self.value
    value = np.array([value], dtype=np.int64)
    r = round_ns(value, rounder, freq)
``` So it shows up on the issue properly: #21262  Cleaned up my implementation here
1) Params added in test cases
2) round_ns now only takes np.array ( hence cleaned up the logic in here )

Thoguhts @mroeschke ? Thanks for the revision. As @jreback mentions, I dont think it's necessary to iterate, You should be able to perform the adjustments with vectorization. At a high level the logic should look like:

```
mask = value % unit == 0
if mask.all():
    return value
values[~mask] = _round_non_int_multiple(values[~mask])
return values
``` That @mroeschke your logic above seems much neater let me refactor!
 Thanks @jreback, ive updated, let me know if there is anything else! Done! @jreback Travis flagged some linting errors:

```
pandas/_libs/tslibs/timestamps.pyx:92:14: E128 continuation line under-indented for visual indent
pandas/_libs/tslibs/timestamps.pyx:92:80: E501 line too long (80 > 79 characters)
pandas/_libs/tslibs/timestamps.pyx:103:80: E501 line too long (87 > 79 characters)
pandas/_libs/tslibs/timestamps.pyx:104:14: E128 continuation line under-indented for visual indent
``` thanks @alimcmaster1  thanks @jreback and @mroeschke for helping review!