QoL: simplify Settings.transient and Settings.final_time

[RemDelaporteMathurin]

FESTIM/festim/settings.py

Lines 63 to 65 in 31864c8

	# TODO maybe transient and final_time are redundant
	self.transient = transient
	self.final_time = final_time

It can be annoying to have to set both the arguments final_time and transient as it is kind of redundant.
If we provide a final time, then obviously the simulation is transient.

We could simplify this by removing the transient argument altogether and replace it by a property attribute of Settings.

@property
def transient(self):
    return self.final_time is not None