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
BUG: DatetimeIndex.asobject raises ValueError when contains NaT
Merge pull request #7544 from sinhrks/dtmixin

BUG: DatetimeIndex.asobject raises ValueError when contains NaT
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