Gravity particles

Ok, that's another big one... This includes all the changes by @tt and @jregan regarding the use of an external potential.

The main changes are:

• Update to the unit with some more const statements.
• Addition of a set of physical constants and of an object to convert them in the chosen system of units.
• A new external gravity task and its dependencies.
• This task calls functions in a new file potential.h where users can define their external potentials.
• A new test case that just makes a bunch of particles orbit a point mass.