product_measurement_type for area_percent

[bdeadman]

Is your feature request related to a problem? Please describe.
LC area percent is a common measurement type in industry HTE. In the recent Pfizer dataset ord-data/#189 I used a custom measurement type for LC area percent. There is a purity type which is probably the most accurate description of what is measured, but LC area percent in the context of HTE is generally interpreted as a surrogate for yield.

Describe the solution you'd like
An area_percent measurement type could avoid suggesting the measurement is a yield or purity, and would be complementary to the area/count/intensity types.

This can be discussed at an ORD meeting.