New features:

1. Allow prep_pp_survival_data to receive pp_data as a dataframe, rather than force the user to use internal functions provided by a list of columns - issue Allow pp-data to be passed to prep_pp_survival_data #57.
2. Support for detailing parameters to SurvivalStanData within a patsy formula, rather than passing as parameters. This is optional, but will make certain features (like supporting competing risks or joint models - issue Implement semi-competing risks / joint model #36 ) down the line cleaner. Is also a prerequisite for allowing more flexible spec of varying-coefs - issue Support specifying which variables to vary by group in fit_stan_survival_model #47.
3. Add iter column to time_betas extract - issue add iter column to time-betas #73

Variety of bug fixes:

1. Fixed incompatibility with pandas >= 0.23.4 (Merging on int64 and object columns, pandas 0.23.4 pandas-dev/pandas#23733) Dataframe Merge error in utils.plot_pp_survival (Test pem_survival_model with simulated data notebook) #76
2. Fixed log_t_dur incorrectly specific Possible log_t_dur misspecification in Stan model file #71
3. Fixed prep_data_long_surv fails on multi-event data #78