algebra: when compileme fails, print operator
algebra: add Parallel

Goes with Sequence and DoWhile
query_test: fix compilation in STDEV case

@7andrew7  this is a hack -- thoughts?
myrialang.compile_to_json: refactor for sequence

Using the new SubPlan-based Myria json format.
myrialang: autoformat fixes from my editor

Sorry. It just happened.
don't add useless label to call compile_to_json
myrialang: remove stray print
DistributedGroupBy issue: improve comment
myrialang.compile_to_json: wrap ops in a Parallel

Even a unary Parallel, because we map it to the JSON "SubQuery" subplan node.
myrial_test: let caller specify the output relation to check

Want this for sequence when I write two different relations.
myrial: test compiling/executing Sequence
myrialang: address comments

mostly fix Python comments to be accurate from wrong ;)
Merge pull request #222 from uwescience/add-myria-sequence

Add myria sequence