[ ] Log-likelihoods taking subject weights as parameters instead of model pars (facilitate IJ, etc.)
[ ] Splines for covariates + penalized splines for covariates and baseline hazards
[ ] Aalen-Johansen
[ ] progress meter
[ ] replace joint optimization with optimization of each hazard separately - can be accomplished via color matrix passed on to Optimization.jl
[ ] infinitesimal jackknife estimate for the variance + cross-validation/model comparison
[ ] Completely semiparametric inference via proportional transition intensity models with weighted partial likelihoods? Does this work? Could leave baseline intensity unspecified and accumulate importance sampling weighted partial likelihood contributions.
[ ] make non-modifying versions of initialize_parameters! and set_parameters!