Improved ground track when number of orbits is 1

[srcejon]

When the number of orbits to display for ground tracks is set to 1, the tracks typically start near the equator, which means they aren't too useful when the satellite is near the equator. E.g: here the satellite is moving north, but the track doesn't show where it is going.

In this branch: #194 I've changed it so that when the number of orbits is less than 2, the ground track starts 25% of an orbit behind the current satellite position, which I think is a bit more useful, when a satellite is near the equator. (Here the ISS is moving East)

Also, I've added support for a fractional number of orbits, from 0.5 to 1.9, so you can get a little bit of a wrap around over the current position, without there being too much clutter of having several complete orbits. E.g. here is it set to 1.4 (so the arrow head I added for #94 happens to be in just the right position to show which way the satellite is moving).