Implement iterative spectrum solver

[david-pl]

This implements a method to compute the spectrum directly in Fourier space using IterativeSolvers together with LinearMaps (similar to steadystate.iterative). Still needs some more tests and polishing.

[codecov]

Codecov Report

Merging #313 (94baebd) into master (7da9e95) will decrease coverage by 0.03%.
The diff coverage is 96.77%.

@@            Coverage Diff             @@
##           master     #313      +/-   ##
==========================================
- Coverage   98.24%   98.20%   -0.04%     
==========================================
  Files          16       16              
  Lines        1308     1339      +31     
==========================================
+ Hits         1285     1315      +30     
- Misses         23       24       +1     

Impacted Files	Coverage Δ
src/timecorrelations.jl	98.61% <96.77%> (-1.39%)	⬇️

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