Modified the string representation of Factor class to have headers as well.
Modified the string representation of Factor class to have headers as well.

anaviltripathi/pgmpy,palashahuja/pgmpy,vivek425ster/pgmpy,jhonatanoliveira/pgmpy,khalibartan/pgmpy,sandeepkrjha/pgmpy,jhonatanoliveira/pgmpy,pgmpy/pgmpy,ankurankan/pgmpy,ankurankan/pgmpy,anaviltripathi/pgmpy,yashu-seth/pgmpy,liquidmetal/pgmpy,sandeepkrjha/pgmpy,abinashpanda/pgmpy,pratyakshs/pgmpy,pratyakshs/pgmpy,kris-singh/pgmpy,liquidmetal/pgmpy,yashu-seth/pgmpy,abinashpanda/pgmpy,khalibartan/pgmpy,palashahuja/pgmpy,pgmpy/pgmpy,kris-singh/pgmpy,vivek425ster/pgmpy