PERF: performance gains in DataFrame groupby.transform for ufuncs (GH7383)

[jreback]

accelerates non-modifying transformations, e.g.
closes #7383

DataFrame.groupby(...).transform(np.max)

-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
groupby_transform_ufunc                      |   6.1977 | 215.6494 |   0.0287 |
groupby_transform2                           | 155.9653 | 155.1824 |   1.0050 |
groupby_transform                            | 167.5134 | 165.7823 |   1.0104 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------

Ratio < 1.0 means the target commit is faster then the baseline.
Seed used: 1234

Target [3d3715b] : WPI: fast tranform on DataFrame
Base   [eb1ae6b] : Merge pull request #7458 from sinhrks/intersection