Allow using only an elastic collision in the cross section file

Goal: Allow the use of an input cross section file with only an elastic cross section.

Problem: When trying to test MultiBolt by calculating the Maxwell model which is defined by a constant collision frequency (and implemented by having a single elastic cross section scaling inversely with the square root of the energy) problems arose. MultiBolt expected excitation and ionization cross sections otherwise s was not set also the matrix A_f and solutions f_0 contained NaNs due to divisions by 0 when constructing A_f due to a 0 ionization coefficient.