nystuen_dacey: Dominant flows
In Geographie-cites/toyspace: bla bla bla

This function selects the flows to be keeped in a large matrix of flows responding to the Nystuen & Dacey's dominants flows criterion.

1 2	nystuen_dacey(tabflows, idori, iddes, idflow, weight, threspct, pol, idpol, centPol, idCentPol, poptab)

`tabflows`	A data.frame of flows between origins and destinations (long format matrix containing, at least, 3 column : origins, destinations, flows)
`idori`	A character string giving the origin field name in tabflows
`iddes`	A character string giving the destination field name in tabflows
`idflow`	A character string giving the flow field name in tabflows
`weight`	A character string, chosen between, "destination", "origin" or "sum" to weigths the flows
`threspct`	A threshold (see 'Details')
`pol`	An sf object of the cities
`idpol`	A character string identifier of cities
`centPol`	A character string identifier of cities
`idCentPol`	A character string identifier of cities
`poptab`	A character string identifier of cities This function selects which flow (fij or fji) must be kept. If the ratio weight of destination (wj) / weight of origin (wi) is greater than the treshold, then fij is selected and fji is not. This function can perform the second criterion of the Nystuen & Dacey's dominants flows analysis. As the output is a boolean matrix, use element-wise multiplication to get flows intensity.