SaVi satellite constellation visualizer savi-dev

upating savi constants dialog to show new formatting, Earth/Mars popup.

Introducing planetary body C-side. Earth is 0, for now Mars is 1.

Also removing 'savi' command, which was duplicate of 'satellites', never used.

savi command as an alternative to satellites was never used. Shorter, though.

Reset to default now obeys planet setting.

there's a lot of cruft C-side around defaults.

Minor cleanup of display variables - they're an overlay params_display().

Improving Simulation constants.

We now call out the length of year, which, like tilt,
is used in pseudo-Sun calculations.

Equator and tropics do alternate pixels. so
if they're together (no tilt) it becomes a
solid line.

floating point safety!

Beefing up setting global body params in Tcl rather than C,
making it easier to support different planetary bodies.

This means that there's some redundant C code that could be
cleaned up in future.

Mars, baby!

Adding a popup planet selector to the Simulation constants dialog.
Currently, only choices are the default Earth, and Mars.

Proof that Mars works is that the clarke geo script computes 20427.68 km
as the stationary orbit radius, which is correct.

Still to do: forcing Mars color shades, turning off Earth maps (inc.
land/sea), even adding Mars maps.

Other planets/bodies could be added - Venus rotating backwards is tricky,
not sure about the Moon's gravitational model.