Helper functions for measurements

[kzollove]

We should have some simple helper functions. Namely, one for value_as_number measurement records.

The simplest of these should append the operator_concept_id to value_as_number for display.

More robust functionality should remove these values from summary statistics.

Out of 610M records in measurement table, 7M records have a value in operator_concept_id field that is not '='.

[clarkevans]

Indeed. What we should do is pair program on that.