Parse forward slashes in gwas catalog

[kshefchek]

For example the row:

2020-06-25	25555482	Gelernter J	2014-09-16	Biol Psychiatry	www.ncbi.nlm.nih.gov/pubmed/25555482	Genome-wide association study of nicotine dependence in American populations: identification of novel risk loci in both African-Americans and European-Americans.	Nicotine dependence symptom count	3,529 African American individuals, 4,117 European American individuals	NA	6p21.32	6	32383959	intergenic	TSBP1-AS1			ENSG00000225914			rs35794310/rs147955325/rs11415565-TG	rs35794310/rs147955325/rs11415565	...

Per https://www.ncbi.nlm.nih.gov/snp/rs35794310 - rs35794310 was merged with rs11415565
and https://www.ncbi.nlm.nih.gov/snp/rs147955325 - rs147955325 was merged with rs11415565

We should model this similarly to how we model deprecated identifiers in ontologies, but it's unclear from this row alone which identifier is the current one (is it always the last in the list?)

See monarch-initiative/monarch-ui#383

[kshefchek]

According to the docs, if MERGED == 1, we should be using the SNP_ID_CURRENT column

Looks like we already have some support for this:
https://github.com/monarch-initiative/dipper/blob/254242e2/dipper/sources/GWASCatalog.py#L450

From the gwas catalog docs:

SNPS*: Strongest SNP; if a haplotype it may include more than one rs number (multiple SNPs comprising the haplotype)

MERGED*: denotes whether the SNP has been merged into a subsequent rs record (0 = no; 1 = yes;)

SNP_ID_CURRENT*: current rs number (will differ from strongest SNP when merged = 1)