memoized get_local_rows

This *appears* correct. I tested it by inspection with the SUGG and CRAM data on a set seed in CV. I think I *could* set up a test for this, but it'd be a lot of work, a time-consuming test, and amount to repeatedly re-doing said test-by-inspection.

As it is, this memoization looks like it cuts the run time of CV down to a quarter of what it was.

refs: https://memoise.r-lib.org/reference/memoise.html#details
https://rdrr.io/r/base/ns-hooks.html
r-lib/profvis#134
r-lib/memoise#79 (comment)

DESCRIPTION

@@ -18,6 +18,7 @@ Imports:
     dplyr,
     lubridate,
     magrittr,
+    memoise,
     purrr,
     rlang,
     scoringRules,

R/zzz.R

@@ -0,0 +1,3 @@
+.onLoad <- function(libname, pkgname) {
+  get_local_rows <<- memoise::memoise(get_local_rows, omit_args = c(".datetime"))
+}