Chabrier IMF in test_lf.py

Added Chabrier IMF, function returns dN for a given mass array.
-inputs: mass_array, min_mass, max_mass, mass_critical, sigma_mass_critical
-Although Chabrier is usually given as dN/dlog10M, the function here
 converts dN/dlog10M to dN/dM since the mass array is input as linear.