CDAP-713: Updated SparkPageRank archetype according to migration from procedures to services.
CDAP-538: Updated scripts to use new services; Minor docs update.
CDAP-538: Updated docs.
Merge remote-tracking branch 'upstream/release/2.6.0' into CDAP-713/SparkPageRank-example-archetype-2.6.0
CDAP-713: Updated docs.
Merge pull request #704 from cybervisiontech/CDAP-538-spark-page-rank-app-2.6.0

CDAP-538: Updated scripts to use new services; Minor docs update.
Merge pull request #703 from cybervisiontech/CDAP-713/SparkPageRank-example-archetype-2.6.0

CDAP-713: Updated SparkPageRank archetype according to migration to services.
make all implementations of getDataSet() throw DataSetInstantiationException
rename DataSetInstantiationException to DatasetInstantiationException (no more camelCase)
rename DataSetContext to DatasetContext (no more camelCase)
rename DatasetContext.getDataSet() to getDataset(); keep old one as deprecated
small simplification
also rename DataSetInstantiator
remove legacy getDataSet() instead of deprecating it
Merge branch 'feature/get-ds-exceptions' into release/2.6.0
Merge branch 'release/2.6.0' into feature/r2.6.0_doc_updates