Check for schema matching for binary bag/set opertions
Test binary bag/set operations
myrial_test: add a few more helper functions

Before, we could only execute queries. Now we can also get plans of
various types directly.
ensure SchemaMismatchException on logical plan
TDD tests for schema issues in while loops
Check for schema compatibility on re-assignment to a variable
Fix a unit test to avoid schema change on assignment
Merge pull request #214 from uwescience/andrew_schema_mismatch

Check for compatible schemas during set operations