Better country code validation

[bbengfort]

In the iso3166 package there is very lightweight country code management for ISO-3166-1 Alpha 2 and 3 codes. However, these codes are not incorporated into legal person and natural person validation in the IVMS 101 code base. Users should be able to specify either a country or a country code and have it correctly modified by the IVMS101 package.

IVMS 101 country code validation is here:

• ivms101.go#L69
• ivms101.go#L130

[bbengfort]

Note that in 03d5820 I added a List method to the iso3166 package to return all of the country codes in the packet as well as a lightweight case-insensitive search method based on prefix to Find.