Scan should only calc min and max if mode is "full"

[agstephens]

• scan should NOT read all the data values by default
  • the default should be "quick" mode
  • It should set the min and max of the main variable to: None
• add a command-line argument called "-m|--mode" that defaults to "quick"
  • it can be set to "full"
  • if mode is "full": then DO calculate the min and max.

[agstephens]

Done.