ENH: resampling functionalities is now applying anti-alias filter

[luca-s]

Reference: http://www.nicholson.com/rhn/dsp.html

[damb]

@luca-s, thanks for the PR.

• May I ask why you decided to implement the lowpass filter by yourself? Is there a reason why you didn't fall back to BW_LP(order, hi-freq) from https://docs.gempa.de/seiscomp/4/4.0.2/base/filter-grammar.html for the anti-alias filter?

• What about implementing unittests?

[luca-s]

* May I ask why you decided to implement the lowpass filter by yourself? Is there a reason why you didn't fall back to `BW_LP(order, hi-freq)` from https://docs.gempa.de/seiscomp/4/4.0.2/base/filter-grammar.html for the anti-alias filter?

I wanted something specific for resampling and faster. Have you thought on how you would use the BW_LP in the code?

* What about implementing unittests?

If you find the time to do so, I will borrow your code ;)