ENH: test_perf.py - use psutil to set affinity (if absent functionality - then affinity module)

original affinity module is not commonly installed, has known issues,
and was not updated since 2006.  psutil is already used in parts of
pandas, so it would be logical to use it instead by default.
minor: some trailing spaces and a pylint "pragma" to stop complaining about Series._ix defined elsewhere
ENH: ujson better handling of very large and very small numbers, throw ValueError for bad double_precision arg #4042
ENH: expose ujson precise_float argument on decode
BUG: explicity change nan -> NaT when assigning to datelike dtypes
Merge pull request #4296 from neurodebian/master

Use psutil to set process affinity (test_perf.py) + minor pylint-friendliness for ._ix
DOC: docs for precise_float option in read_json
Merge branch 'Komnomnomnom-ujson-precise-float'
Merge pull request #4302 from jreback/nat_convert

BUG: explicity change nan -> NaT when assigning to datelike dtypes
Merge pull request #4299 from Komnomnomnom/ujson-small-floats

ENH: ujson better handling of very large and very small numbers, throw ValueError for bad double_precision arg #4042