Added back exponential term

Signed-off-by: Ian bell <ian.h.bell@gmail.com>
Added SAFT term

Signed-off-by: Ian bell <ian.h.bell@gmail.com>
Added Lemmon2005 HE term and added checks to avoid calculations of logs for other terms

Signed-off-by: Ian bell <ian.h.bell@gmail.com>
doubles -> long double

Signed-off-by: Ian bell <ian.h.bell@gmail.com>
Started adding PT flash and SRK - need to think about data structures

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