allow overlap for fft segments

[S-Hanly]

I may have steered @theflanman wrong here when I asked for this function but forcing the overlap to 0 results in more noisy results and doesn't seem necessary. Here's an example using the existing function

Here it is when we allow for overlapping which is the default scipy behavior

[codecov-commenter]

Codecov Report

Merging #194 (deb7d8b) into development (64800ff) will decrease coverage by 0.01%. The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           development     #194      +/-   ##
===============================================
- Coverage        69.10%   69.09%   -0.02%     
===============================================
  Files               34       34              
  Lines             2965     2964       -1     
===============================================
- Hits              2049     2048       -1     
  Misses             916      916              

Impacted Files	Coverage Δ
endaq/calc/fft.py	72.14% <100.00%> (-0.20%)	:arrow_down:

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[S-Hanly]

Thanks!