add argument to allow negative slopes

[temospena]

In order to know if a linestring has a positive or negative slope (considering direction), add an argument to slopes to allow retrieving slopes as negative, instead of always absolute.
..., allow_negative=T

[temospena]

or
..., directed = TRUE
or
..., directed = "oneway"
It could work with the info of an existing column of oneway - usually part of a road network.

once that we can get z0 and z1 with slopes we can say that if z0 > z1, the slope would be -1*slope
and with the information of "FT" (1x) and "TF" (-1x).
and for those with "both"? duplicate the segment to store both information?

This is useful for routing, and not so much for visualisation.

(I might need some help to make this happen 🔢 )