Deprecate welch_pgram without explicitly given window

[martinholters]

In preparation of #152/#153 to be done in v0.9.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.84%. Comparing base (3c1b783) to head (eeb56d5).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #581   +/-   ##
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  Coverage   97.84%   97.84%           
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  Files          19       19           
  Lines        3252     3255    +3     
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+ Hits         3182     3185    +3     
  Misses         70       70           

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[martinholters]

Due to the short length of x, the default n==3 and

julia> hanning(3)
3-element Vector{Float64}:
 0.0
 1.0
 0.0

is not at all better than the default rect window. In the next example, where n=5 is explicitly set, I've changed the window to hanning as it somewhat reasonable there. I think from a pedagogical viewpoint this is ok, and I'd like to keep a complete rewrite of the example out of the scope of this PR.

Sidenote: a variant of the windows without the zeros at the edges might a useful here, e.g.

julia> hanning(5)[begin+1:end-1]
3-element Vector{Float64}:
 0.5
 1.0
 0.5

But that's also beyond the scope of this PR.

[martinholters]

CC @JakeZw