Adjust the way monster upgrade times are calculated. [Balance]

src/monster.cpp

@@ -233,7 +233,7 @@ void monster::hasten_upgrade() {
// Checking for return value of -1 is necessary.
int monster::next_upgrade_time() {
    const int scaled_half_life = type->half_life * ACTIVE_WORLD_OPTIONS["MONSTER_UPGRADE_FACTOR"];
    int day = 0;
    int day = scaled_half_life;
    for (int i = 0; i < UPGRADE_MAX_ITERS; i++) {
        if (one_in(2)) {
            day += rng(0, scaled_half_life);

src/options.cpp

@@ -1124,7 +1124,7 @@ void options_manager::init()
                                   );
    OPTIONS["MONSTER_UPGRADE_FACTOR"] = cOpt("world_default", _("Monster evolution scaling factor"),
                                    _("A scaling factor that determines the time between monster upgrades. A higher number means slower evolution. Set to 0.00 to turn off monster upgrades."),
                                    0.0, 100, 1.0, 0.01
                                    0.0, 100, 4.0, 0.01
                                   );

    mOptionsSort["world_default"]++;