ULXs luminosity functions

[AArn80]

In this pull request, I submit the functions that I have developed to calculate the X-ray luminosity of XRBs. It includes two way of calculating these luminosities: using the King et al. (2001, 2009) prescriptions and a new set of GRRMHD prescriptions. This code was developed for my Master's thesis work under the supervision of @maxbriel and @tassos25 . Tassos Fragos asked me to create a pull request with my functions as well as a small notebook on how to use my functions. Here are the files:

1. To calculate the Bondi-Hoyle-Lyttleton mass accretion rate, I am using the wrong version mentioned in the issue Incomplete calculation of the Bondi-Hoyle accretion rate #829 .
2. "Luminosity_XLF_functions26-05-28.py" contains all the functions used to calculate and plot the cumulative X-ray luminosity functions.
3. "XLF_Guide_functions "(.HTML or .ipynb version (I recommend to download the .ipynb version in order to have the correct layout)) contains some explanations and example on how to use the functions that I have implemented.