Correct marginal pm1 for mass model

• Added \frac{1}{m_1-m_{min}} factor
• Allow user to choose primary powerlaw index \alpha (can be either float or 'salpeter') and m_{min} (has to be smaller than m_{min,bank}). Defaults are 'salpeter' and 0.8
• salpeter_prob kept as a function that calls primary_powerlaw with hardcoded alpha=2.35
• changed name of manifold_cbc_bank_salpeter_mass_model to manifold_cbc_bank_mass_model
• Organize mass-model related info in yaml