[BUG] Uncontrolled sample rate changes with downsampling

[m-nishim0t0]

Need to address downsampling issue with logLira in unitFilt.

[FrancescoNegri]

When running doUnitFilter, each channel goes through the following steps:

1. Stimulus artifacts rejection (optional)
2. Filtering
3. Downsampling (optional)

If downsampling is enabled, the Block.SampleRate is modified after the first processed channel and the subsequent channels believe that new frequency is their original sample rate. This causes isssues in the conversion of stimulus onsets from time to samples and also in the filtering step, when setting the filter characteristics.