Improve voltage accuracy by averaging

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What steps will reproduce the problem?
1. Connect a voltage sensor to APM
2. Configure MinimOSD to show voltage
3. Turn everything on, aircraft on the ground, and look through the FPV link. 
Read the voltage reading.

What is the expected output? What do you see instead?
I would expect to see a relatively stable voltage, which should be in line with 
what Mission Planner is showing (and close to the true one).

Instead, I get highly fluctuating values that can be as far as 0.25V off the 
real value. In total I get about 0.5V fluctuation.

What version of the product are you using? On what operating system?
Using 2.4 r727 of the firmware. APM 2.5.2.

Please provide any additional information below.
I have troubleshooted the problem (latest developments documented here: 
http://ardupilot.com/forum/viewtopic.php?f=25&t=11227) and have concluded that 
this is not APM's fault, but the OSD's. The values the OSD gets from the 
MavLink stream should be averaged, to filter out variations. 

Mission Planner does this, and its reading is both accurate (compared to real 
voltage) and stable (0.05V deviation). Only drawback I can see is that it takes 
time to register big changes. E.g. when you initially power up, it takes about 
30" for the average to reach the real voltage. No biggie.

It is very important (at least for me) to get a readable, easy to understand 
value when flying, without having to average out values in my mind. I believe 
this improvement will give a more "professional feel" to the OSD and 
consequently APM, as the stable readings give a feeling of reliability and 
dependability, at least for me.

Original issue reported on code.google.com by cma...@gmail.com on 11 Feb 2015 at 9:18