Btyd

- add `legend` argument to `(m)bgcnbd.PlotTrackingInc`, `(m)bgcnbd.PlotTrackingCum`, `mcmc.PlotTrackingCum`, and `mcmc.PlotTrackingInc`
- made vignette code compatible with BTYD 2.4.2
- deprecated inefficient alternative sampling method for Pareto/NBD MCMC
- updated referenced URLs (to HTTPs or redirected pages)

- `elog2cbs` handles now empty elogs gracefully (58)

- limit simulated life length to 10k events in `xbgcnbd.GenerateData`

- fix `mcmc.ExpectedCumulativeTransactions` for Pareto/NBD (Abe) with covariates (55)
- add ability to provide covariates to `abe.generateData`

- added `sales.x` to `elog2cbs` output, which sums over sales in calibration, but excludes initial transaction (thanks to msinjin for the suggestion)

- added `(m)bgcnbd.PlotFreqVsConditionalExpectedFrequency` methods
- added `(m)bgcnbd.PlotRecVsConditionalExpectedFrequency` methods