ENH: Adding support for streaming and priority determination in. These changes to VTK improve the way that sources can provide meta-information about pieces of the data. It also changes VTK so that the request_update_extent_information_pass propagates the meta-information down the pipeline over only those filters that do not modify the corresponding information. The pass also gives filters a chance to compute a priority from the meta-information, which the sink can then use to reorder or skip pieces that do not matter.