[feature] Added BRENDA Tissue Ontology
Reworked several attributes inherited from ICGC

Attributes specimen_type, specimen_type_other and specimen_source were
inherited from ICGC, but they were too focused on cancer projects.

They have been replaced by tissue_type (direct mapping of IHEC
TISSUE_TYPE), tissue_depot (direct mapping of IHEC TISSUE_DEPOT) and
specimen_term, which maps to BRENDA Tissue Ontology the information
provided by the two previous attributes.
Assembly metadata update

Added specific optional columns for Ensembl and Gencode versions
used.
Added GRCh37.p10 to the controlled vocabulary.
Model v0.3.1.0