Implement probability mass function (PMF)

[mmeierer]

Add the functionality to compute and plot the probability mass function (PMF) for the

• [x] Standard BG/NBD model
• [x] Standard Pareto/NBD model
• [ ] Extended Pareto/NBD model
• [ ] GGompertz/NBD model (no mathematical derivation exists yet)

[mmeierer]

Status 11-08-2020 (contributions of @niels89):

• generic methods pmfund clv.model.pmf created,
• parts of PNBD PMF implemented

See branch feature-pmf.