Indexing / search for R and Python interface

[bschiffthaler]

Implement calls to SeidrFileIndex::get_offset_node and SeidrFileIndex::get_offset_nodelist. R and Python interfaces could then call e.g.:

src <- c('gene_a', 'gene_b', 'gene_c')
tgt <- c('gene_x', 'gene_y', 'gene_z')
edges <- find_pairwise(sf, src, tgt, symmetric = FALSE)
# yields a->x, b->y, c->z
# if symmetric, also yield x->a, y->b, z->c

src  = ['gene_a', 'gene_b', 'gene_c']
tgt = ['gene_x', 'gene_y', 'gene_z']
edges = sf.get_pairwise(src, tgt, symmetric=False)