KernelGenerator with empty FieldSet

[erikvansebille]

The Kernel generator now needs a FieldSet, which sometimes hampers code development. @CKehl has already fixed this in #862, these changes need to be brought over to a separate PR.

In order to make it work, @CKehl needed to switch the parameter order from KernelGenerator(fieldset, pyfunc) to
KernelGenerator(pyfunc, fieldset) (next to some other adaptations). Hence after manually merging the two files, we may need to adapt some KernelGenerator calls in the ParticleSet, the Kernel and/or the RNG to the new order.

This will also help fix #618. Thanks to @CKehl for bringing this up