v0.7.5

• direct value access for DataRow object.

  DataRows now directly access the respective values from the columns.
  This improves runtime and memory footprint for most DataFrame operations.
  DataRow objects are invalidated once the source DataFrame is changed.
  Accessing an invalidated row results in an exception

• Row collections are now return as DataRows object.
  DataRows can be converted to a new DataFrame

• improved 'groupBy' method