Update Measurement.value_as_number guidance

[MelaniePhilofsky]

The Measurement.value_as_number guidance states "If there is a negative value coming from the source, set the VALUE_AS_NUMBER to NULL, with the exception of the following Measurement" and then lists 7 LOINC codes. However, Loinc code 18517-3, QRS axis, can be negative. There are probably other LOINC codes from EKG readings which can be negative since parts of an EKG can have negative values.

These data come across in EHR "Lab" tables.

[ericaVoss]

@MelaniePhilofsky - I added the QRS axis to the list but was successful in finding EKGs to include.

@aostropolets or @dimshitc does LOINC provide any information on which codes are allowed to have negative values? I'm looking for a more standardized way to identified measurements that allow negative values. You can see what I'm doing here: https://github.com/OHDSI/Themis/issues/16

[clairblacketer]

This was added to the THEMIS convention library and CDM documentation was updated accordingly