Add option to eigensolver to look at time derivative operator

Gives the option of SLEPc looking at the time derivative operator (approximated by g_{n+nadv}-g_{n} / (code_dt nadv)) instead of the usual time advance operator. The SLEPc eigenvalue for the time advance operator is exp(-i*omega*dt*nadv) whilst the SLEPc eigenvalue of the time derivative operator is -i*omega.

The main advantage of the time derivative operator is that the specified tolerance now relates directly to omega rather than exp(-i*omega*dt*nadv). It may also make some of the SLEPc options a little more intuitive in places.