DO April 18 2019 release

This release contains (doid.obo file): 9,262 disease terms, 98% with a single is_a parent, 6158/9262 (66%) of terms defined. This release includes the addition of ICD-O xrefs, inferred Y-linked inheritance disease terms, updating of the OMIM susceptibility terms, addition of pediatric cancer terms for ClinGen, addition of disease subtypes (e.g. Meier-Gorlin syndrome, miliaria), new DO terms (e.g. thiamine deficiency syndrome), adding back into DO Equivalent To statements that had disappeared during the file processing, reinstating Nipah virus encephalitis and the addition of OMIM, GARD, Orphanet and MESH IDs based on Wikidata updates.