Explicitely use and import dplyr::n

Fixes issues with dplyr 1.0

Change-Id: I73c59b00ee8fcc525084767d7520d6dfc4afb311
diff --git a/NAMESPACE b/NAMESPACE
index 096d676..3596a04 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -13,6 +13,7 @@
 export(hc_freq_by_year_ci)
 export(ipm)
 export(mutate)
+export(n)
 export(percent)
 export(queryStringToLabel)
 export(select)
@@ -49,6 +50,7 @@
 importFrom(dplyr,enquo)
 importFrom(dplyr,group_by)
 importFrom(dplyr,mutate)
+importFrom(dplyr,n)
 importFrom(dplyr,rename)
 importFrom(dplyr,rowwise)
 importFrom(dplyr,select)