Fixed some bugs relating to transfer of computed correlators between twopoint objects.
Fixed calculation of standard deviation in jackknife - added sqrt
Added jackknife specific error function
Fixed bug in dataset resampling methods: average now performed before measurement
Added subtract and negative functions to Observable-based classes
Fixed bug in BareTwoPoint.__add__: output created as BareTwoPoint
Fixed bugs in new neg and subtract functions.
Improved efficiency of jackknife: subtract bins from sum
Added square brackets overload to dataset.
Added function to update datum with new value.
Added function to apply a function to each item in the dataset.
Added index argument to getitem function in DataSet