Select observations to retain by specifying a function
    The specified function is evaluated on each obesrvation.
 
  selectedDs = ds.select(selectFunction);
 
  There are two ways to define selectionFunction
    One input, One logical vector output
        selectFunction recieves the input data set and must
        output a nObservations by 1 logical vector.
    One input, One logical scalar output
        selectFunction recieves the ObservatioinInfo structure
        of a single observation.
 
  Examples:
    ds = prtDataGenIris;
    ds = ds.setObservationInfo(struct('asdf',num2cell(randn(ds.nObservations,1))));
 
    dsSmallobservationInfoSelect = ds.select(@(ObsInfo)ObsInfo.asdf > 0.5);
 
    dsSmallObservationSelect = ds.select(@(inputDs)inputDs.getObservations(:,1)>6);