Improve quality of DataSet spec tests

Whenever possible, it is good to be specific in tests.
Update insight.DataSet.group to consider its filter function

Introduce test to cover this
Add documentation for Grouping.postAggregation
Update changelog to explain the issue fixed
added conversion utility and function for arff to json data conversion
Merge pull request #296 from matthewdunsdon/feature/dataset-group-consider-filters

Update insight.DataSet.group to consider its filter function
Merge pull request #297 from matthewdunsdon/feature/document-insight.Grouping.postAggregation

Add documentation for Grouping.postAggregation
changed error handling to callback and minor tweaks as per review
further updates, used node style parameter ordering
fixed typo
updated changelog
Merge pull request #298 from seanworkcode/feature/arff-converter

added conversion utility and function for arff to json data conversion
Updated changelog version number