Copter: Added enabled(bool) function to AP_RangeFinder_analog class
-to be able to Enable and Disable with Mavlink/GCS because _enabled flag is Private in class.
Copter: Analog Sonar Support

Added sonar support using the AP_Rangfinder_analog class instead of the
Maxbotix class.  This brings to same as arduplane and ardurover.  This
class is much nicer because we can tweek sonar values through GCS
parameters.  Downside is now sonar selection is not trivial, may need to
implement simple config on GCS side or add a wiki for default settings
of most common analog sonars.
-Added enabled(bool) setter function to AP_Rangefinder_analog in order
to dynamically enable and disable sonar at runtime with chan7 or chan 8 options
-Fixed sonar_alt and sonar voltage reports to mavlink GCS, now displays
in mission planner correctly.
-User should set max_distance in parameters to appx 60% of max for
reliability reasons, because built in 60% is removed. (Less is better)
-Added voltage and filtered report to CLI Test Sonar
-Added Mode filter to read_sonar funtion to help filter out and smooth random spikes.

*Tested on Pixhawk, sonar pin 15 maxbotix MB1240XL, scale factor 2.42
*Not tested on APMx (I don't have one)