Add Hernquist and NFW potential to SWIFT

configure.ac

doc/RTD/source/ExternalPotentials/index.rst

doc/RTD/source/index.rst

examples/Hernquist_circularorbit/hernquistcirc.yml

examples/Hernquist_circularorbit/makeIC.py

examples/Hernquist_circularorbit/plotprog.py

examples/Hernquist_circularorbit/run.sh

examples/Hernquist_radialinfall/README

examples/Hernquist_radialinfall/hernquist.yml

examples/Hernquist_radialinfall/makeIC.py

examples/Hernquist_radialinfall/plotprog.py

examples/Hernquist_radialinfall/run.sh

examples/IsothermalPotential/energy_plot.py

examples/IsothermalPotential/isothermal.yml

examples/NFW_Halo/README

examples/NFW_Halo/makeIC.py

examples/NFW_Halo/makePlots.py

examples/NFW_Halo/run.sh

examples/NFW_Halo/test.yml

examples/parameter_example.yml