advanced error checking

[alessandrofelder]

Things that would be nice to have would be

• an early check that all user specified files exist (so the user doesn't wait for a large base grid to be read only to find out there is a typo in the source grid file they specified)
• an early check that the source grid region overlaps (entirely? partially?) with the base grid
  Not a priority.
• check for non-compatible datums
• filter any "non-geographical" values (<<Marianna trench, >> Mt Everest)