head method
Returns a DataFrame consisting of the first nRows rows or less if the instance
holds less.
asView set to true leads to a view of the current data being returned, otherwise
a copy will be returned.
Implementation
DataFrame head({int nRows = 5, bool asView = true}) => _viewOrCopy(
_trackedColumnNames.l, getRange(0, min(nRows, length)).toList(), asView);