RCG: Add a 'time_shift' parameter to ADC blocks in models.

'time_shift' is an integer. For fast and low-noise ADC, 'time_shift' can range from 0 - clock_div (usually clock_div is 8).

Typically an IOP will read a fast or low-noise ADC 'clock_div' times per cycle. With this change, the IOP will take 'time_shift' values from the previous cycle and

'clock_div' - 'time_shift' values from the current cycle, freeing more time to do other things in teh current cycle, but adding a bit of latency to the data.

For 16 bit ADC, time_shift can be 0 or 1. When 1, ADC uses the value from the previous cycle.

In any case, 'time_shit' 0 causes the IOP to read all data from the current cycle. For fast and low-noise ADC this can take the whole cycle.

Closes #296 (closed)