Lazy index for fixed length windower

[PierreGtch]

This adds the option to use lazily compute the trial bounds when using create_fixed_length_windows.

Useful when working with small strides that can lead to metadata DataFrames nearly the same size as the dataset itself.

[codecov]

Codecov Report

Attention: Patch coverage is 97.33333% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 87.37%. Comparing base (c409800) to head (e773410).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #597      +/-   ##
==========================================
+ Coverage   87.25%   87.37%   +0.11%     
==========================================
  Files          67       67              
  Lines        5980     6051      +71     
==========================================
+ Hits         5218     5287      +69     
- Misses        762      764       +2     

[bruAristimunha]

@robintibor, can you review this modification on temple university dataset?

[bruAristimunha]

very small comments, but lgtm

[PierreGtch]

Thanks @bruAristimunha for the comments! :)

[bruAristimunha]

Thank you @PierreGtch =)