Added ability to output matrix as string with formatting

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Added derivatives from Gernert; Added Catch tests for Mixture derivatives; Accept "Methane|Ethane" style for Mixture instantiation

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Updates to changelog and phase_envelope.ipynb

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