Add stellar orbital periods

Add stellar orbital periods as described in GURPS Space p. 115. A
subcompanion for the second companion of the main star is not accounted
for in these calculations.
Add planetary orbital periods

Add the calculation of orbital periods for planetary bodies and
asteroid belts.
Add planetary orbital eccentricity generation

Add the automated generation of planetary orbital eccentricities. For
the sake of completeness, also asteroid belts get an orbital
eccentricity. This can be interpreted as the mean eccentricity of the
asteroid belt.
Calculate min and max separation in OrbitContent

When the eccentricity gets delivered to an OrbitContent, it
automatically calculates the minimum and maximum separation from the
primary star.
Add orbit generation for Moons

Moons will check the type of their parent planet and generate their
orbit around it accordingly. Currently they do not check that the orbit
generated is valid in terms of proximity to other satellite objects,
mainly other Moons.
Add generation of Moonlet orbits

Moonlets will now accept an optional positional keyword argument called
family, which is switched to 'first' or 'third' for Moonlets of gas
giants and defaults to None.
Add satellite orbital periods

Satellites will generate their orbital periods according to the formula
presented in the errata for GURPS Space p. 117.
Add total tidal effect to terrestrial planets

Terrestrial planets will now calculate their TTE as per GURPS Space
p. 117. The corrected formula from the errata was used.
Add total tidal effects to Moons

Moons will now calculate their TTE.
Add rotation periods for terrestrial planets

The rotation period is given in days. If it is negative the rotation is
retrograde.
Add rotation period for Moons
Add local calendar calculations for Moon
Sort Moons according to orbit when generating them
Add local calendar for Terrestrial Planets

Terrestrial planets will now calculate the apparent length of the sun
cycles and moon cycles.
Add generation of random axial tilt

Terrestrial planets will now generate an axial tilt as described in
GURPS Space p. 118.
Merge branch 'dynamics'

Close #8