Make atomic rates return zero if plasma and/or beam parameters <= 0

[vsnever]

As suggested in #386 (comment), there is no need to set near-zero thresholds for plasma and beam parameters in atomic rates. Instead of this:

core/cherab/openadas/rates/pec.pyx

Lines 58 to 64 in d033dd1

	# need to handle zeros, also density and temperature can become negative due to cubic interpolation
	if density < 1.e-300:
	density = 1.e-300
	if temperature < 1.e-300:
	temperature = 1.e-300

atomic rates should return zero if the parameters are less than or equal to zero:

    if density <= 0 or temperature <= 0:
         return 0

[vsnever]

Fixed in #451.