Change PerfomanceBaseConversion descriptor name to reflect that it may not be a conversion

The name PerfomanceBaseConversion implies that there is a conversion, and in many cases there is not.  If there is a number or letter grade, those may very well translate to "passing" "failing" "mastery" etc.

But in many cases, there is not actually a conversion going on. In some cases (and you can see values on DATASTD-1146) , there are just values like "Exceeds expectation" or "Mastery" etc. This is really just a "Performance Level" (per the assessment domain entity) and not really a conversion.