Add likelihood function to main.py

The function to calculate likelihood (Eqn 2) now takes as
inputs 1-D arrays and not matrices. The arrays are for a
fixed i (one data point) and varying k (isochrone points)
so that the previously used loop over k can be eliminated.
This should speed up the likelihood calculations.

Current likelihood function includex exponential argument
and constant factor 1/2pi*sqrt(det). rho(Z,k) and c(S/N)
have not been implemented yet.