[ENH] improve `freq` parameter in edc module

[f-brinkmann]

Checklist

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Question

The freq parameter is used in four functions and can be 'broadband' or a single frequency.

• Do we want to enable passing array likes? Otherwise it would be cumbersome to compute RA parameters in (fractional) octaves
• Do we want to rename the parameters? freq seems a little generic
• Do we want to improve its documentstion? I did not understand, what the parameter does, before checking the code.

[mberz]

I think these are all valid points.

• We do however would need to think about a proper matching strategy for the input dimensions of freq and the number of channels in the input data.
• On the renaming: Something indicating that the frequency controls a smoothing parameter is a good idea :)

[f-brinkmann]

we decided to rename freq to smoothing_parameter, which can except 'broadband' a scalar frequency or an array like of frequencies. Coulde later be extended to exepts a ufunc as well.

If an array like is passed it's length must equal signal.cshape[0]

[mberz]

In case this does not make it into v1.0.0, I propose to move this into 1.1.0 and deprecate the old parameter and remove it in 1.3.0