Master

Expanded the derivatives case to calculate Q (second invariant of grad(u,v,w)) and R (third invariant/determinant of grad(u,v,w)). These quantities are useful for detecting coherent structures in turbulent flows. To implement these changes the following files were created:

1. SPINS_main/src/Science/Q_invt.cpp
2. SPINS_main/src/Science/R_invt.cpp

Additionally, the following files were modified (details in the commits):

1. SPINS_main/src/Science.hpp
2. SPINS_main/src/cases/derivatives/derivatives.cpp
3. SPINS_main/src/cases/derivatives/spins.conf
4. SPINS_main/src/VERSION

I updated the version number to 2.1.6 (incrementing by 1 minor version), although this may be on the order of a patch ¯_(ツ)_/¯.