Extract several helper methods from CF checker into util module
Check geospatial/temporal attribute extents vs data in ACDD, fixes #10

- geospatial_lat_min/geospatial_lat_max checked against potential lat vars
- geospatial_lon_min/geospatial_lon_max checked against potential lon vars
- geospatial_vertical_min/geospatial_vertical_max checked against potential vertical vars
- time_converage_start/time_coverage_end checked against potential time vars

UdUnits and dateutil needed for converting time units. All variables identified via CF recommendations.
Merge pull request #48 from daf/acdd_extents

Check geospatial/temporal attribute extents vs data in ACDD