mavextra: correct radian conversion in distance_lat_lon function

[shancock884]

fix #635
As pointed out in issue #635, there is an error in the distance_lat_lon function in mavextra, as cos(lat1) and cos(lat2) were being applied to the value in degrees not radians.
This PR fixes this, using the code proposed by @gitdillo, by ensuring all function inputs are converted before use.

I chose not to use the code from the gps_distance function in mp_util.py (as linked @khancyr), as the mp_util function calculates the rhumb line distance, rather than the great-circle distance, and I did not want to alter the original author's intent of the function.

I also added extra words to the comments of all 3 similar functions in mavextra.py and the radius_of_earth variable, to make it more clear which formula and earth radius are being used.

I checked a few spot points before and after against the Movable Type Scripts website. For example:
Before fix:

>>> import pymavlink.mavextra
>>> pymavlink.mavextra.distance_lat_lon(51,0,53,2)
125501.69695239903

After fix:

>>> import pymavlink.mavextra
>>> pymavlink.mavextra.distance_lat_lon(51,0,53,2)
261134.12945358222

Movable Type Scripts website: 261.1 km (to 4 SF)

[tridge]

thanks!