project_rows()
results in a weighted one-mode object
that retains the row nodes from a two-mode object,
and weights the ties between them on the basis of
their joint ties to nodes in the second mode (columns).
project_cols()
results in a weighted one-mode object
that retains the column nodes from a two-mode object,
and weights the ties between them on the basis of
their joint ties to nodes in the first mode (rows).