spflow_model_matrix: Generate design matrices for the spatial interaction model
In spflow: Spatial Econometric Interaction Models

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
)

`sp_multi_network`	A `sp_multi_network()` object that contains information on the origins, and the destinations and their neighborhood structure
`network_pair_id`	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)