Added function to generate time-varying noise.

[sgaebel]

I put the function in it's own file with an example / a test case in the "if name == 'main':" block. Currently, it does not use any of the pycbc infrastructure, editing the spots where I use seperate frequency & gain/amplitude arrays to use the FrequencySeries class may be a good next step, but I'm not very familiar with the pycbc internals.

[ahnitz]

@sgaebel Can you look at how we've done this in https://github.com/gwastro/pycbc/blob/master/pycbc/noise/reproduceable.py and see if you can re-use some of the same functions and style there? Hopefully you can see how the filter is calculated and applied there. We certainly would want to avoid any duplication.

[sgaebel]

Is this better? (also, some of the automated check seem to be stuck. no idea if I can do anything about those)

[spxiwh]

Is this still useful? I note that #3949 claimed to do something similar, but that doesn't look it truly produces realistic non-stationary noise, and I think that this does (having not looked over the code in any detail).

[ahnitz]

I think someone will need to pick this up again, so closing for now.