SaVi satellite constellation visualizer savi-dev

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.

Commenting on the ugliness of an old hack that
still needs fixing.

Updating manual to cover new equator and tropics options.
Also mentioning tilt in Simulation constants, which should
have been mentioned with the 1.6 release.
Updating screenshots to suit.

minor code tidyup.

Adding in menu option for tropics flag.

fixed drawing of tropics for spherical/cylindrical projections,
which need -all-maps flag to even see.