ENH: Implement linspace behavior for timedelta_range and interval_range
- [X] closes #20976
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

Summary:
- added linspace behavior support to `timedelta_range`
- added linspace behavior support to `interval_range`
    - refactored `test_interval_range.py` to use `parametrize`; split up some tests
- added a check to `bdate_range` to ensure that `freq` is specified
    - doesn't really make sense to support linspace behavior since `bdate_range` implies a frequency
    - same underlying code as `date_range`; could get linspace behavior by overriding `freq` to `None`
- updated and cleaned docstrings for all `*_range` functions except `period_range`
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21009?src=pr&el=h1) Report
> Merging [#21009](https://codecov.io/gh/pandas-dev/pandas/pull/21009?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/52effcb6848c05226bfdc103e28beb01d972c63f?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21009/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21009?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21009      +/-   ##
==========================================
+ Coverage   91.82%   91.82%   +<.01%     
==========================================
  Files         153      153              
  Lines       49491    49505      +14     
==========================================
+ Hits        45446    45460      +14     
  Misses       4045     4045
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.22% <100%> (ø)` | :arrow_up: |
| #single | `41.88% <77.77%> (+0.02%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21009?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/21009/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `93.14% <100%> (+0.05%)` | :arrow_up: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/21009/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.77% <100%> (+0.01%)` | :arrow_up: |
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/21009/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `91.24% <100%> (+0.08%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21009?src=pr&el=continue).
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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21009?src=pr&el=footer). Last update [52effcb...a220176](https://codecov.io/gh/pandas-dev/pandas/pull/21009?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 nice patch @jschendel !

Ideally we could add a small doc-update in timeseries: http://pandas-docs.github.io/pandas-docs-travis/timeseries.html#generating-ranges-of-timestamps

and in the timedelta / interval sections if appropriate.