Vectorize probabilities in `FlavorTransformation`

[Andrey Sheshukov]

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.