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.

See merge request !143