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 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 36 ) down the line cleaner. Is also a prerequisite for allowing more flexible spec of varying-coefs - issue 47.
3. Add `iter` column to time_betas extract - issue 73
Variety of bug fixes:
1. Fixed incompatibility with pandas >= 0.23.4 (https://github.com/pandas-dev/pandas/issues/23733) 76
2. Fixed log_t_dur incorrectly specific 71
3. Fixed 78