Introduced new composite datasets for partitioned data.

This is the first step in our attempt to create next generation composite datasets for partitioned data and arbitrary dataset collections. The main idea is that partitioned datasets are instances of vtkPartitionedDataSet, which is a vector of datasets. It is required that these datasets are compatible, i.e. can be processed by the same type of filters together. This is similar to vtkMultiPieceDataSet but will formalize that partition concept further. The second new data type is vtkPartitionedDataSetCollection. This is a vector of vtkPartitionedDataSets. It can be an arbitrary collection but is not a tree like its predecessor vtkMultiBlockDataSet. Any further grouping of the blocks will be done by the use of SIL data structures, which will be supported later.

Also added basic IO support for these data types.