Add clarification note to gender property in BioSample profile

[AlasdairGray]

Add Bioschemas description to the gender property in the BioSample profile to state that it should be used to capture the biological sex of the sample.

Due to the nature of schema.org it does not seem appropriate to make the distinction between biological sex and gender.

[JonnyJD]

In a later internal meeting it was mentioned that this is most accurate the "genetic sex" in these circumstances.