Document reason mapping all gp clinical to measurement

[MaximMoinat]

https://github.com/EHDEN/ETL-UK-Biobank/blob/master/docs/gp_clinical_to_stem_table.md

Reason: Read maps to condition concepts. But there is a value.

[MaximMoinat]

While working on the documentation, also address that the waist/hip circumference are observations: #176