Salpeter IMF in test_lf.py

Added f_salpeter. The function takes in:
- a mass array
- a min mass below which dN/dM = 0
- a max mass above which dN/dM = 0
- alpha = abs val of exponent of linear version of Salpeter, recall alpha=Gamma+1
- calculates dM between each consec elements of mass array
- returns dN = dN/dM * dM array