Almost done with third partial derivatives for associating fluids

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Fixed citation to close https://github.com/ibell/coolprop/issues/165

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
More work on partials in AssociatingFluids.ipynb

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>