Better documentation needed for get/setCustomParameters

[kmdouglass]

These methods are part of the ImageGenerator interface but it's not quite clear what they're used for from the Javadocs.

[MStefko]

Thinking about it, these methods have no business being part of the interface. These methods should be removed, and any implementation should be refactored into class-specific methods. E.g. if an implementation uses setCustomParameters("laser_power", value), it should be refactored into its own method setLaserPower(value).

[MStefko]

See #66