user-specified binning

[nhejazi]

Via the grid_type argument to haldensify(), which is passed to format_long_hazards(), discretization and binning of the outcome variable (A) in the conditional density estimation routine g(A|W) is handled by functions that bin points based on an equal partitioning along the range of A ("equal_range") or into possibly uneven bins that contain roughly the same number of observations each ("equal_mass"), as happens at https://github.com/nhejazi/haldensify/blob/master/R/utils.R#L51-L63. It would be beneficial in many applications to allow the user to specify the binning/discretization themselves, given that these two simple routines may lead to suboptimal performance in many settings, e.g., when there are rare instances that fall far away from others among the set of observations. It would be beneficial to allow the relevant argument to take as input a set of cut points along the observed support at which the variable A will be discretized.