fixing LanczosLowNoise

[ranocha]

This is an extended version of #288 including some required fixes. Thanks a lot to @reula for preparing the initial draft!

Closes #288
Closes #287

[codecov]

Codecov Report

Attention: Patch coverage is 95.83333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 91.84%. Comparing base (f49dad4) to head (bbf702e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/periodic_operators.jl	95.83%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #290      +/-   ##
==========================================
+ Coverage   91.80%   91.84%   +0.03%     
==========================================
  Files          36       36              
  Lines        5530     5578      +48     
==========================================
+ Hits         5077     5123      +46     
- Misses        453      455       +2     

Flag	Coverage Δ
unittests	91.84% <95.83%> (+0.03%)	⬆️

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[coveralls]

coverage: 92.56% (+0.03%) from 92.535%
when pulling bbf702e on reula_lanczos
into f49dad4 on main.