make ops.add_foo take just class
As discussed

Also small cleanup for Sparse ops.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19828?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@92dbc78`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19828/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/19828?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #19828   +/-   ##
=========================================
  Coverage          ?   91.68%           
=========================================
  Files             ?      150           
  Lines             ?    48943           
  Branches          ?        0           
=========================================
  Hits              ?    44875           
  Misses            ?     4068           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.06% <100%> (?)` | |
| #single | `41.86% <81.53%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19828?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/19828/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `96.94% <100%> (ø)` | |
| [pandas/core/sparse/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/19828/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL2ZyYW1lLnB5) | `94.81% <100%> (ø)` | |
| [pandas/core/sparse/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/19828/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL3Nlcmllcy5weQ==) | `95.25% <100%> (ø)` | |
| [pandas/core/sparse/array.py](https://codecov.io/gh/pandas-dev/pandas/pull/19828/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL2FycmF5LnB5) | `91.32% <100%> (ø)` | |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/19828/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.41% <100%> (ø)` | |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/19828/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.23% <100%> (ø)` | |
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/pull/19828/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `97.3% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19828?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19828?src=pr&el=footer). Last update [92dbc78...45467f1](https://codecov.io/gh/pandas-dev/pandas/pull/19828?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
