Merge branch 'GravityParticles' into 'master'
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