Read distance from/to planets from the space platform hub

[eiennohito]

TL;DR

Add an option to read the distance to/from the currently navigated planet to the specified signal.

What?

We can read speed of the space platform, but the option to read distance would be also very helpful. The value is already in game.

Why?

While the distance is sort of possible to compute in theory by integrating speed with the time (using a clock circuit), reading the actual value to the circuit would be much easier to use. For example to decrease speed in areas with high asteroid density.