Test the presence of from and to colnames

[agila5]

As described in #271, we currently observe the following cryptic error message when creating a sfnetwork object without the necessary from and to columns:

library(sf)
#> Linking to GEOS 3.11.2, GDAL 3.7.2, PROJ 9.3.0; sf_use_s2() is TRUE
library(sfnetworks)

p1 = st_point(c(7, 51))
p2 = st_point(c(7, 52))
l1 = st_sfc(st_linestring(c(p1, p2)))

edges = st_as_sf(data.frame(X1 = "A", X2 = 1, X3 = 2), geometry = l1, crs = 4326)
nodes = st_as_sf(c(st_sfc(p1), st_sfc(p2)), crs = 4326)

(sfn <- sfnetwork(nodes, edges, message = FALSE))
#> Error in (function (edges, n = max(edges), directed = TRUE) : At rinterface_extra.c:82 : The value nan is not representable as an integer. Invalid value

^{Created on 2024-07-12 with reprex v2.0.2}

After the commit included in this PR, we see the following

library(sf)
#> Linking to GEOS 3.11.2, GDAL 3.7.2, PROJ 9.3.0; sf_use_s2() is TRUE
library(sfnetworks)

p1 = st_point(c(7, 51))
p2 = st_point(c(7, 52))
l1 = st_sfc(st_linestring(c(p1, p2)))

edges = st_as_sf(data.frame(X1 = "A", X2 = 1, X3 = 2), geometry = l1, crs = 4326)
nodes = st_as_sf(c(st_sfc(p1), st_sfc(p2)), crs = 4326)

(sfn <- sfnetwork(nodes, edges, message = FALSE))
#> Error: Edges are spatially explicit, but 'from' and 'to' columns are missing

edges = st_as_sf(data.frame(X1 = "A", from = 1, to = 2), geometry = l1, crs = 4326)
(sfn <- sfnetwork(nodes, edges, message = FALSE))
#> # A sfnetwork with 2 nodes and 1 edges
#> #
#> # CRS:  EPSG:4326 
#> #
#> # A rooted tree with spatially explicit edges
#> #
#> # A tibble: 2 × 1
#>             x
#>   <POINT [°]>
#> 1      (7 51)
#> 2      (7 52)
#> #
#> # A tibble: 1 × 4
#>    from    to X1            geometry
#>   <int> <int> <chr> <LINESTRING [°]>
#> 1     1     2 A         (7 51, 7 52)

^{Created on 2024-07-12 with reprex v2.0.2}

A couple of comments:

1. I can include a proper unit test if you think that this change is a good idea;
2. I'm not 100% sure that we should error in this case, maybe a warning is enough... The reason is that the underlying tidygraph code may work even if the edges table does not contain any column which is named from and to:

https://github.com/thomasp85/tidygraph/blob/382c2b7eec121b7bd842b8acb4209367b4943d74/R/list.R#L72-L75