exponential_decay: `t_1_e` error propagation

[pathfinder49]

I'm pretty sure the divide by 2 shouldn't be there...

oitg/oitg/fitting/exponential_decay.py

Lines 28 to 29 in 26e047e

	# This is just a Gaussian error propagation guess.
	p_err["t_1_e"] = np.sqrt(p_err["x0"]**2 + (p_err["tau"] / 2)**2)