Fixed inability to disable dziFormat

[jmschrack]

-d / --dziFormat was hardcoded to set dziFormat to true. dziFormat is already defaulted to true. Changed to -d takes boolean true/false. In sliceImage "if [ ! dziFormat ] " was testing if the variable was unset on some shells. Changed to string comparison for greater portability.

[VoidVolker]

This option is reserved for second mode for non dzi format - this mode not very good tested and currently dzi is shall be default mode.