refactor assignment syntax in simdrive_impl

[kylecarow]

Noticed the syntax on assignments to simdrive arrays could be cleaner, so I tidied those up.

Before:

if condition_1 {
    LHS = value_1;
} else if condition_2 {
    LHS = value_2;
} else {
    LHS = value_3;
}

After:

LHS = if condition_1 {
    value_1
} else if condition_2 {
    value_2
} else {
    value_3
};

Also prevented recalculating dt_s() many times in set_post_scalars

[calbaker]

Also prevented recalculating dt_s() many times in set_post_scalars

Does this mean it's faster?

[kylecarow]

Does this mean it's faster?

@calbaker I would assume so, but I didn't benchmark it. It's also in a function that's only run once per sim_drive() call