update frost vignette

vignettes/frost.Rmd

@@ -27,6 +27,17 @@ df <- friendlyr::ts(service="frostlyr",username="****",password="****",
                  fromDate=fromDate,toDate=toDate,precision=precision,timeResolution=timeResolution,
                  element_id=c("sum(precipitation_amount PT1H)"),station_id="SN18700")
 
+
+# get data over an area
+
+element_id=c("sum(precipitation_amount PT1H)",
+             "air_temperature",
+             "relative_humidity")
+
+df <- friendlyr::ts(service="frostlyr",username="****",password="****",
+                 fromDate=fromDate,toDate=toDate,precision=precision,timeResolution=timeResolution,
+                 element_id=element_id,location="oslo,Norway")
+
 ```
 ## Get metadata