Convert computed band

If a product contains a virtual band such as a virtual band created by a reader and the band has not been loaded by the user then, the imageInfo will be null and the conversion will fail without any error message.

This fix only checks imageInfo for null and warns the user. 
Todo: If the conversion action requires the data to be loaded then it should automatically load the data.

Also, why the new term "Computed Band" when everywhere else it's referred to as a "Virtual Band"?
thanks