dw_crude_mx             Computes crude/raw death rates/probabilities
                        using double rolling windows
dw_preprocess           Pre-processes a data frame of deaths, exposed
                        to risk or times of exposition to be used by
                        the 'dw_crude_mx' function
graduate_qx             Graduates raw death probabilities
tw_crude_mx             Computes crude/raw death rates/probabilities
                        using triple rolling windows
