Vectorize probabilities in FlavorTransformation

[Sheshuk]

Currently FlavorTransformation.prob_ee and similar methods take time and energy as arguments.
These arguments can be arrays of values:

snewpy/python/snewpy/flavor_transformation.py

Lines 39 to 52 in 08c3b84

	def prob_ex(self, t, E):
	"""X -> e neutrino transition probability.
	Parameters
	----------
	t : float or ndarray
	List of times.
	E : float or ndarray
	List of energies.
	Returns
	-------
	float or ndarray
	Transition probability.

However the probabilities are often output as scalars, or as vectors only in E.

I suggest making the output universal: it should always be an array of shape (len(t), len(E)) (or scalar if both are scalars), even if the probability doesn't depend on E or t. This will make it easier for vectorized calculations, and more clear to the user.