DRILL-491: project pushdown into scan. Add base test class for testing plan.

Project pushdown.  Add a plan base test class.  Add a clone method for GroupScan.

Code clean up.

Code clean up.

Code clean up.

Code clean up.

code clean up .

Minor change to DrillScanRel.getGroupScan().

Project push down : skip trivial project.

Enable the rule to remove trivial project in logical planning phase.