Allow fast and low noise adcs to be read earlier

IOP models wait until an entire 15 microseconds' worth of data is read from a fast adc before processing. This leads to problems since the iop doesn't have enough time to turn around and write out DAC values.

One approach is to take fast ADC data that's earlier.

I propose a 'shift' parameter that can be added to a fast ADC block. 'shift' is the number of samples that the IOP ought to read from the previous 15 microsecond time period.

'shift' can be any integer between 0 and UNDERSAMPLE, where UNDERSAMPLE is the number of values read in 15 microseconds. When 0, you get the default behavior that might impact DAC. When UNDERSAMPLE (typically 8), all the points come from the previous 15 usec. The ADC read returns immediately, and there's plenty of time to write to DACs.