Replace survival and hazard function calls with standsurv

[dom-muston]

The standsurv post-estimation command of flexsurv is not currently used, although psm3mkv already depends on flexsurv. Therefore some code in psm3mkv is redundant or inefficient.

Go through code and replace calls to calc_surv() and calc_haz().
Ideally also remove convert_fit2spec().