Speed up routing

[Robinlovelace]

As raised by @mem48 here nptscot#1

[Robinlovelace]

Relevant PR: https://github.com/cyclestreets/cyclestreets-r/pull/45/files

[Robinlovelace]

Code in most recent commit linked to above shows that it is ~2x quicker.

> fromPlace = sf::st_as_sf(lwgeom::st_startpoint(od))
> toPlace = sf::st_as_sf(lwgeom::st_endpoint(od))
> library(sf)
> bench::mark(check = FALSE, max_iterations = 1,
+             journey = (r1 <<- route(l = od, route_fun = journey, plan = "quietest")),
+             journey2 = (r2 <<- journey2(fromPlace, toPlace, id = od$route_id, plan = "quietest", segments = TRUE))
+             )
# A tibble: 2 × 13
  expression      min   median `itr/sec` mem_alloc `gc/sec` n_itr  n_gc total_time result memory     time       gc      
  <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl> <int> <dbl>   <bch:tm> <list> <list>     <list>     <list>  
1 journey       9.16s    9.16s     0.109    20.8MB    0.328     1     3      9.16s <NULL> <Rprofmem> <bench_tm> <tibble>
2 journey2      4.73s    4.73s     0.211    24.4MB    0.423     1     2      4.73s <NULL> <Rprofmem> <bench_tm> <tibble>
Warning message:
Some expressions had a GC in every iteration; so filtering is disabled. 

[Robinlovelace]

Quick win to speed-up reading:

cyclestreets-r/R/journey.R

Lines 306 to 325 in 9449bec

	vals_variable$elevation_mean = stringr::str_split(vals_variable$elevations, pattern = ",") %>%
	lapply(as.numeric) %>%
	lapply(mean) %>%
	unlist()
	vals_variable$elevation_start = stringr::str_split(vals_variable$elevations, pattern = ",") %>%
	lapply(as.numeric) %>%
	lapply(utils::head, 1) %>%
	unlist()
	vals_variable$elevation_end = stringr::str_split(vals_variable$elevations, pattern = ",") %>%
	lapply(as.numeric) %>%
	lapply(utils::tail, 1) %>%
	unlist()
	vals_variable$elevation_max = stringr::str_split(vals_variable$elevations, pattern = ",") %>%
	lapply(as.numeric) %>%
	lapply(max) %>%
	unlist()
	vals_variable$elevation_min = stringr::str_split(vals_variable$elevations, pattern = ",") %>%
	lapply(as.numeric) %>%
	lapply(min) %>%
	unlist()