Add post script to calculate pastro

Write a postscript to generate p_astro

• Modify the p_astro() function in p_astro.py to be a class that accepts initial configurations that include rates of bbh, bns, and nsbh, and the mass that defines the boundary between NS and BH, ...
• Add load_samples() method to RapidPE_result class that load the likelihood sample hdf5 file generated by @vinaya.valsan