Cryoscope: add support for multiple IIR filters

[RoyStegeman]

#1565 introduces a boolean for iir. If True, a single IIR filter will be determined before the FIR taps, if False only FIR taps will be determined (the number of FIR taps can be 0).

For the setups I have tested (platinum and tuna5), the exponential pre-distortions are on very short time scales. Here I'm ignoring the long time-scale distortions since I'm interested in SZN (see also #1235). Because of this, I had no use for exponential filter nor the ability to properly test them.

However, of course the protocol should support the possibility to determine multiple exponential filters. To resolve this issue the iir flag should be replaced by an int indicating the number of filters.

[alecandido]

Adaptive number of exponentials

There is also the option to adaptively determine the number of exponentials. But it seems quite tricky to get it right.

Filters replacement

Another problem is that filters may have multiple origins. E.g. #1256 is introducing a further one.
This is solved by representing them as a list, and applying them in cascade.

While there is no problem in this approach, when they are determined one single time, it introduces the further complication of the list management. This becomes relevant when the filters are updated to replace existing ones.

At the moment, this could be solved by wiping the previously determined one. But with multiple origins, this is not really a viable option, since it would make them mutually exclusive.

I'm not suggesting to face this problem immediately, and we can rely on manual cleaning for as long as needed. But it won't be comfortable.