Made modifications to the AMCurrentAmplifier superclass. Removed pure virtual requirement for sensitivity-related methods (increase/decreaseSensitivity(), atMinimum/MaximumSensitivity(), etc). Added supportsSensitivity/GainMode() functions to declare what modes are available, making both false in AMCurrentAmplifier. Added gain-related methods (increase/decreaseGain(), etc) that depend on whether gain is supported (same with sensitivity).
Simplified interface by removing extra ...Implementation() functions.
Ooops, added the virtual keywords where they were forgotten.
Modified CLSKeithley428 class to conform with new superclass requirements.