Two Green's function convolution

[lxvm]

This PR adds a solver for computing a convolution of two Green's function separated by a momentum q. Example is added to the demos folder

Tests will fail because this requires lxvm/FourierSeriesEvaluators.jl#9

TODO:

• Tests
• PR

cc: @HugoStrand

[codecov]

Codecov Report

Attention: Patch coverage is 31.11111% with 31 lines in your changes missing coverage. Please review.

Project coverage is 49.75%. Comparing base (0d2e9f8) to head (54e695f).

Files with missing lines	Patch %	Lines
src/2GreensSolver.jl	0.00%	29 Missing ⚠️
src/TransportDistributionSolver.jl	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
- Coverage   50.53%   49.75%   -0.79%     
==========================================
  Files          23       24       +1     
  Lines        1777     1807      +30     
==========================================
+ Hits          898      899       +1     
- Misses        879      908      +29     

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