Replaced mu variables by mu_Earth and mu_Sun variables

[Arunim10]

After seeing all the python codes in this orbit determinator, I thought that initialising a variable mu for storing gravitational parameter of Earth and sun , was a creating much confusion. So I replaced all the variables mu ,in which the gravitational parameters of Earth and sun was stored, by mu_Earth and mu_Sun respectively.

First Commit : "replace mu by mu_Earth"

You can see in the changed files that I have changed all the variables mu by mu_Earth and mu_Sun wherever required and by doing this , I have avoided the confusion of having mu in many places.
I have also kept the units of mu_Earth and mu_Sun uniform throughout the whole orbitdeterminator program i.e.. , km^3/sec^2 for mu_Earth and au^3/day^2 . This avoided the problem of changing the units of these value in many places.
For instance,
In this python file orbitdeterminator/kep_determination/least_squares.py, previously in line 463, mu_Earth was assigned its value in m^3/sec^2 units but I changed it into units of km^3/sec^2 to maintain uniformity and then wherever mu_Earth was needed in terms of m^3/sec^2 units , I simply multiplied mu_Earth(km^3/sec^2) with 10**(9) to convert it into m^3/sec^2 units.

Second Commit : "Replace mu variable by mu_Earth variable"

I had written some rough code so as to check that whether the code was running fine , so in this commit , I have commented those codes.