custom distribution for flexsurvreg

##' ## Custom distribution
##' ## make "dEV" and "pEV" from eha package (if installed)
##' ##   available to the working environment
##' if (require("eha")) {
##' custom.ev <- list(name="EV",
##'                       pars=c("shape","scale"),
##'                       location="scale",
##'                       transforms=c(log, log),
##'                       inv.transforms=c(exp, exp),
##'                       inits=function(t){ c(1, median(t)) })
##' fitev <- flexsurvreg(formula = Surv(futime, fustat) ~ 1, data = ovarian,
##'                     dist=custom.ev)
##' fitev
##' lines(fitev, col="purple", col.ci="purple")
##' }


##'
##' ## Custom distribution: supply the hazard function only
##' hexp2 <- function(x, rate=1){ rate } # exponential distribution
##' hexp2 <- Vectorize(hexp2)
##' custom.exp2 <- list(name="exp2", pars=c("rate"), location="rate",
##'                     transforms=c(log), inv.transforms=c(exp),
##'                     inits=function(t)1/mean(t))
##' flexsurvreg(Surv(futime, fustat) ~ 1, data = ovarian, dist=custom.exp2)
##' flexsurvreg(Surv(futime, fustat) ~ 1, data = ovarian, dist="exp")
##' ## should give same answer