This function returns the absolute path/to/PIN.sif. While the default PINs are
'Biogrid', 'STRING', 'GeneMania', 'IntAct', 'KEGG' and 'mmu_STRING'. The user can also
use any other PIN by specifying the 'path/to/PIN.sif'. All PINs to be used
in this package must formatted as SIF files: i.e. have 3 columns with no
header, no row names and be tab-separated. Columns 1 and 3 must be
interactors' gene symbols, column 2 must be a column with all
rows consisting of 'pp'.