Document TLE's derived values

[stevenpaligo]

There are four derived values for TLEs and Latest TLEs: period, SMA, apogee height, perigee height. They may not be calculated how the user expects, so they should be documented. The following formulas come from the SpaceTrack website:

• period = 1440/n
• Using mu, the standard gravitational parameter for the earth (398600.4418), semi-major axis "a" = (mu/(n2pi/(24*3600))^2)^(1/3)
• Using semi-major axis "a", eccentricity "e", and the Earth's radius in km, apogee = (a * (1 + e))- 6378.135
• Using semi-major axis "a", eccentricity "e", and the Earth's radius in km, perigee = (a * (1 - e))- 6378.135

Perhaps those TLE-related classes would include getters for apogee/perigee radius as well.

[stevenpaligo]

The getters for the radius values were added in version 1.1.0.