required changes to check_bb() limits or error message.

[cmahony]

the check_bb() limits are quite tight and users can inadvertently exceed them. see #284

i used google earth to find the precise limits of North american extent. southern border of Mexico is 14.53N, northern limit of arctic is 83.118N, Western extent of aleutian islands extends past -180E. so the only problem is that the western extent does exclude outlying aleutian islands.
I recommend we see if we can expand the extent slightly to accommodate user data, if climr source data allows. if not, we should clarify the error message such as "input fields lon and lat are not in lat/long coordinates, or extent is outside ext(-179.0625, -51.5625, 14.375, 83.125)"

ideally, we address this at same time as #179