add sample rate setting

[Michael Welsch]

the 5-pol analog filter in the hardware is optimized for the 48kHz samples per second. anyways, if the user wants a lower sample rate, the original signal has to be digitally filtered before downsampling the values. this should be implemented in the driver.

this here seems to be a good starting point.
https://github.com/terrygta/SignalResampler

if we resample block-wise, there are artifacts at the start and the beginning due to the math behind resampling and filters, so it needs paddings from original data to overcome this problem. Here you take larger blocks (adding pads), apply resampling, and then cut the additional pad information in the result.

[Ingo Kaiser]

@iluxa we should add new method therefore. with resulting fps in Start callback then - default is 48k.

[Ilya Gavrilov]

Initial implementation is on the branch issue_45
implementation is based on https://github.com/terrygta/SignalResampler
with padding

tested on RPi without triangular signal - CPU CORE usage is incremented about 10-15%
Future activities: test with triangular signal; memory copiyng optimizations

[Ilya Gavrilov]

Tests with triangular signal:

[Ilya Gavrilov]

First without resample
Second with 24K resample
Third with 5K resample

[Ingo Kaiser]

@mkh-welsch expected result?

[Michael Welsch]

looks like the instability, we try to avoid with the padding. try to increase the padding size.

[Michael Welsch]

@iluxa : maybe dynamically increase the padding size with lower resample rate.

[Michael Welsch]

effect of not using paddings during resampling.

top: original without resample
middle: resampled without using padding logic
bottom: resampled with padding.

[Ilya Gavrilov]

faec130
dynamic padding size is implemented

[Ilya Gavrilov]

80c31d1
constant size slices

[Ilya Gavrilov]

PR for merge
#62