Fixed feature generation

-added breaks to the feature generation switch statement; forgot to do
it  before. It fixes the issue where the length of the string would be
greater than 4
-added the 4th character to the string; I added rivers/lakes there
instead of 3rd because ice needs to be a feature but i generate it
before (could change, maybe later)
-Updated instructions to warn user that regenerating maps would take up
alot of ram
-Updated comments in hex class to make it just river and lakes, instead
of all the possible combinations of rivers