Implement parsing of MVP IEC

[NOP0]

Need #35 , #33 , #34

Rough sketch, there could be errors here...

PROGRAM Blinky

    VAR
        clock: RTC;
        start: DT# := DT#2019-12-17-00:00:00.000; // Some arbitrary start time
        current: DT#;
        previous: DT#;
        elapsed: T#;
        previous: T#;
        valid: bool;
        period: REAL;
    END_VAR

    IF %IB4 = 0 THEN
        period := 1.0;
    ELSE
        period := 1.0 / %IB4; // %IB4 contains frequency. Byte is promoted to real?
    END_IF;

    // The stuff that follows could also be solved more elegantly 
    // with a duty-timer circuit.
    // But what's below maps more closely to the blinky.rs example

    clock(IN:=TRUE, PDT:=start_time, Q=>valid, CTD=>current_time); // Get current time

    elapsed := elapsed + SUB_DT_DT(IN1:=current_time, IN2:=previous);
    previous := current;

    // REAL_TO_TIME takes msecs    
    IF elapsed > REAL_TO_TIME(period*1000.0) THEN 
        elapsed := T#0;
        %QX0.0 := NOT %QX0.0;
    END_IF;

END_PROGRAM