:bug: Currently unable to run a fully Deuterium plasma without ZeroDivisionError

[kj5248]

Describe the bug

If in an input file you try to set the fuel fraction to only D:

f_plasma_fuel_deuterium = 1.0
f_plasma_fuel_tritium = 0.0

you end up with ZeroDivisionError in different places such as:

• cost model for crfcdr = (fefcdr * self.data.costs.discount_rate) / (fefcdr - 1.0e0) in section related to "Costs due to partial current drive system renewal".
• physics model for self.calculate_electron_ion_collision_time where nd_plasma_ions is 0

[mkovari]

While it would be nice to fix this bug, I would remind you that a fusion plasma with just D is not physically possible, since DD fusion generates tritium half the time. Putting in a small amount of T is therefore not only numerically convenient but physically sensible.