Lifelines

- corrected behaviour in `CoxPHFitter` where `score_` was not being refreshed on every new `fit`.
- Reimplentation of `AalenAdditiveFitter`. There were significant changes to it:
- implementation is at least 10x faster, and possibly up to 100x faster for some datasets.
- memory consumption is way down
- removed the time-varying component from `AalenAdditiveFitter`. This will return in a future release.
- new `print_summary`
- `weights_col` is added
- `nn_cumulative_hazard` is removed (may add back)
- some plotting improvemnts to `plotting.plot_lifetimes`

- More `CoxPHFitter` performance improvements. Up to a 40% reduction vs 0.16.2 for some datasets.

- Fixed `CoxTimeVaryingFitter` to allow more than one variable to be stratafied
- Significant performance improvements for `CoxPHFitter` with dataset has lots of duplicate times. See https://github.com/CamDavidsonPilon/lifelines/issues/591

- Fixed py2 division error in `concordance` method.

- Drop Python 3.4 support.
- introduction of residual calculations in `CoxPHFitter.compute_residuals`. Residuals include "schoenfeld", "score", "delta_beta", "deviance", "martingale", and "scaled_schoenfeld".
- removes `estimation` namespace for fitters. Should be using `from lifelines import xFitter` now. Thanks usmanatron
- removes `predict_log_hazard_relative_to_mean` from Cox model. Thanks usmanatron
- `StatisticalResult` has be generalized to allow for multiple results (ex: from pairwise comparisons). This means a slightly changed API that is mostly backwards compatible. See doc string for how to use it.
- `statistics.pairwise_logrank_test` now returns a `StatisticalResult` object instead of a nasty NxN DataFrame 💗
- Display log(p-values) as well as p-values in `print_summary`. Also, p-values below thesholds will be truncated. The orignal p-values are still recoverable using `.summary`.
- Floats `print_summary` is now displayed to 2 decimal points. This can be changed using the `decimal` kwarg.
- removed `standardized` from `Cox` model plotting. It was confusing.
- visual improvements to Cox models `.plot`
- `print_summary` methods accepts kwargs to also be displayed.
- `CoxPHFitter` has a new human-readable method, `check_assumptions`, to check the assumptions of your Cox proportional hazard model.
- A new helper util to "expand" static datasets into long-form: `lifelines.utils.to_episodic_format`.
- `CoxTimeVaryingFitter` now accepts `strata`.

- bug fix for the Cox model likelihood ratio test when using non-trivial weights.