Creates model matrices from a sp_multi_network() network.
For efficiency the relational representation of origin and destination data
is preserved.
spflow_model_matrix(
sp_multi_network,
network_pair_id,
formula_parts,
estim_control
)A sp_multi_network() object that contains information on the origins,
and the destinations and their neighborhood structure
A character indicating the id of a sp_network_pair() (only relevant if
the sp_multi_network() contains multiple sp_network_pair-objects:
defaults to the of them)
A list of design matrices for spatial interaction model