Allow csteps to specify default source/sink shapes.
TextInputFormat dseq defaults to :vals source shape.
AvroKeyInputFormat dseq defaults to :keys source shape.
AvroKeyOutputFormat dsink defaults to :keys sink shape.
Allow in-memory dseqs to specify default source shape.
Update some existing tests to use default source/sink shapes.
Merge pull request #5 from llasram/default-shapes

Allow dseqs/dsinks to provide default source/sink shapes.