Add SUM and MEDIAN calculation support
Align changes with existing tests, add new SUM and MEDIAN test
Only compute with first field
Remove initialization, check for null in calculation to prevent recursive glitch
Update tests to account for missing fields, handle median values correctly
Remove events sorting, handle scalar values, resolve undesired PQ modifications
Merge pull request #18 from projecttacoma/qdm_functions

Add SUM and MEDIAN calculations from QDM v4.1.1