Added _get_bool_str(value) in two locations.  We need to find single location for it.
Changes boolean based get and set methods to use this.
Added _get_bool_str(value) method to override default with "on"/"off"
Removed functions that were reimplemented to deal with booleans.
Created self test function compatible with Rigol.
Updated voltage values to allow setting outputs to true max range of power supply.
Moved limit set above enable, to keep from tripping out the output if enabled on a lower value, before changing.