Full implementation of Galactic Habitable Zone

This version fully implements the Galactic Habitable Zone as a specified distribution. It also allows Observer objects to be placed inside the same spatial distribution as the Transmitters.

The Transmitter object is also more flexible in the properties of its emitted radiation, allowing decay according to an arbitrary power law. This is essential for modelling e.g. gravitational wave signals, which decay as 1/r rather than 1/r^2.