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.

doc/RTD/source/ExternalPotentials/index.rst

+.. External potentials in SWIFT
+   Folkert Nobels, 25th October 2018
+   
+External Potentials 
+===================
+
+SWIFT can be run with an external potential on this page we will summarize the
+current potentials which can be run with SWIFT and how to implement your own 
+potential in SWIFT.
+
+Implemented External Potentials
+-------------------------------
+
+Currently there are several potentials implemented in SWIFT. On this page we 
+give a short overview of the potentials that are implemented in the code:
+
+1. No potential (none)
+2. Point mass potential (point-mass): classical point mass, can be placed at
+   a position with a mass.
+3. Plummer potential (point-mass-softened): in the code a softended point mass 
+   corresponds to a Plummer potential, can be placed at a position with a mass.
+4. Isothermal potential (isothermal): An isothermal potential which corresponds 
+   to a density profile which is :math:`\propto r^{-2}` and a potential which is 
+   logarithmic. This potential has as free parameters the rotation velocity 
+   and the position.
+5. Hernquist potential (hernquist): A potential that is given by the Hernquist 
+   potential: 
+   
+   :math:`\Phi(r) = - \frac{GM}{r+a}.`
+
+   The free paramters of Hernquist potential are mass, scale length,
+   and softening. The potential can be set at any position in the box.
+6. NFW potential (nfw): The most used potential to describe dark matter halos, the  
+   potential is given by:
+
+   :math:`\Phi(r) = - \frac{4\pi G \rho_0 R_s^3}{r} \ln \left( 1+ 
+   \frac{r}{R_s} \right).`
+
+   This potential has as free paramters the concentration of the DM halo, the
+   virial mass (:math:`M_{200}`) and the critical density.
+7. Sine wave (sine-wave)
+8. Point mass ring (point-mass-ring)
+9. Disc Patch (disc-patch)
+
+
+How to implement your own potential
+-----------------------------------
+
+The first step in implementing your own potential is making a directory of your
+potential in the ``src/potential`` folder and creating a file in the folder 
+called ``potential.h``.
+
+Configuring the potential 
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+To get started you can copy a ``potential.h`` file from an already implemented 
+potential. In this potential the header guards (e.g. ``#IFDEF <>``) need to be 
+changed to the specific potential and the ``struct`` and 
+``potential_init_backend`` need to be  changed such that it uses your potential 
+and reads the correct potential from the parameter file during running the 
+program.
+
+Add the potential to the ``potential.h`` file in the ``src`` directory such that
+the program knows that it is possible to run with this potential.
+
+Furthermore during the configuration of the code it also needs to be clear for 
+the program that the code can be configured to run with the different 
+potentials. This means that the ``configure.ac`` file needs to be changed.
+This can be done to add an other case in the potential::
+
+  case "$with_potential" in
+     none)
+        AC_DEFINE([EXTERNAL_POTENTIAL_NONE], [1], [No external potential])
+     ;;
+     newpotential)
+        AC_DEFINE([EXTERNAL_POTENTIAL_NEWPOTENTIAL], [1], [New external potential])
+     ;;
+
+After this change it is possible to configure the code to use your new potential.
+