ENH: Adding a NotImplementedError for simultaneous use of nrows and chunksize.

For read_csv() the user intention most likely is to get a TextFileReader, when using the chunksize option, but simultaneous use of nrows is not implemented yet. This raises now a NotImplementedError. Test and entry to current whatsnew source (v0.14.1.txt) added.
Fixes #6774
PERF: vbench for mixed groupby with datetime (GH7555)

PERF: perform coercing and casting on groupby agg by blocks
Merge pull request #7085 from michaelaye/add_notimp_error

adding a NotImplementedError for simultaneous use of nrows and chunksize...
Merge pull request #7560 from jreback/groupby_first_perf

PERF: vbench for mixed groupby with datetime (GH7555)
API: add/clarify numpy ufunc prepare/wrap
Remove unused variable assignment
Add notes deprecating in_ipnb and in_qtconsole
Merge pull request #7499 from takluyver/ipy-no-detect-frontend

Deprecate detection of IPython frontends
DOC: Add cookbook recipe like R's `expand.grid()`

Add `expand_grid()` recipe to cookbook

Add note about cookbook recipe to release notes

Revert "Add note about cookbook recipe to release notes"

This reverts commit e431a024c2e688f9917cafb12235ee72ecc3e119.

Remove pandas import, print dataframe in cookbook
Merge pull request #7556 from onesandzeroes/expand-grid

DOC: Cookbook recipe for emulating R's expand.grid() (#7426)
BUG/PERF: perf issues in object groupby aggregations (GH7555)
Merge pull request #7568 from jreback/perf2

BUG/PERF: perf issues in object groupby aggregations (GH7555)
WIP: select_dtypes impl