`line_segment` fails when `segment_length` is specified

[Aeilert]

Hi,

Thanks for a very cool package!

I think there might be a bug in your line_segment() function when using the segment_length argument.

E.g.

> l <- routes_fast_sf[2, ]
> l_split <- line_segment(l = l, segment_length = 500)
Error in Ops.units(n_segments, 1) : 
  both operands of the expression should be "units" objects

The cause appears to be that round(l_length/segment_length) returns an object in units , which causes n_segments + 1 to fail.

https://github.com/ropensci/stplanr/blob/8a9c7b5a9309c48a542e157205fdaff1ddee120e/R/linefuns.R#L157-L164

Coercing n_segments to a numeric should fix this.

  if (!is.na(segment_length)) {
    l_length <- sf::st_length(l)
    n_segments <- as.numeric(round(l_length / segment_length))
  }