Fix 'is biomarker for' relationship by either renaming it or removing it when not appropriate

[pnrobinson]

The current is_biomarker relationship is almost always picking out items that are not biomarkers for the disease. Can we consider renaming the relation type to something that is less committal.

• co_citation (minimal commitment)
• co_mention_in_abstract
• altered_gene_expression_in_disease (in my opinion the maximal commitment that is still correct for the entries I have seen)