restrict station_metadata to GHCN-M v3
replace Methodology with Method
default meta to a dictionary
remove comparison to GHCN v2
remove redundant imports
remove valid blah methods
replace sys.maxint with None
remove caching, make good_count a method
remove useless StationMetaData class
remove _v2_sources because it is not used
use builtin .count() method in good_count()
remove and revise documentation