Add Hernquist and NFW potential to SWIFT

I added a Hernquist and NFW potential to the code, also I changed the parameters a bit, so that now the use of the position is more consistent. This means that I have an additional parameter useabspos, that when 0 the positions are based on the centre and the offset while if useabspos is 1, the positions are absolute. This way all the potentials behave exactly the same (prevents confusion). Furthermore I added 3 examples, two for the Hernquist potential and one for the NFW profile. Further I added a start of the documentation for the external potentials.