Investigate EventCode implications

Created by: apcarp

I recently found a bug in myquery that stemmed from a possible misunderstanding of mya event codes. I discussed with Chris Slominski how to interpret event codes. Turns out the first and second nibbles of the event code are orthogonal. 0x00, 0x01, ... 0x05 are codes for why there is not data in this update (0x00 for there is data) and 0x00, 0x10, ..., 0x30 are for why there is not data prior to this point (0x00 for there is data prior to this point).

The main point is that any 0x*0 event code will have a valid data value associated with it. That's different than just 0x00 having a data point to use. This will probably impact a variety of methods and structures like streams and downstream dependencies.

myquery issue: https://github.com/JeffersonLab/myquery/issues/11