Merge pull request #7313 from cpcloud/fix-single-group-extract

BUG: single group series should preserve group name
BUG: do not check that val != val and not equal inf/-inf since by def val not eq val is nan
Merge pull request #7315 from cpcloud/fix-dropna-with-inf-7314

BUG: isnull doesn't properly check for inf when requested
implement additional tests for groupby apply methods
simplify groupby's std method
add sem to nanops
add sem to aggregate methods
add sem to groupby
add sem to docs