DLR algorithm for the electron density

[lxvm]

This pr adds an extension for Lehmann.jl that implements an algorithm to compute the electron density on the imaginary frequency axis. This will close #39 when finished. The remaining questions for this method are how to specify the calculation of the imaginary frequency data of the self energy from the real frequency data. While a Hilbert transform will do this, it seems like there should be a new api.

[codecov]

Codecov Report

Attention: Patch coverage is 48.14815% with 56 lines in your changes missing coverage. Please review.

Project coverage is 42.90%. Comparing base (ca7ab3b) to head (f804027).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/self_energies_io.jl	0.00%	49 Missing ⚠️
ext/AutoBZLehmannExt.jl	87.71%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
+ Coverage   42.07%   42.90%   +0.82%     
==========================================
  Files          21       23       +2     
  Lines        1666     1755      +89     
==========================================
+ Hits          701      753      +52     
- Misses        965     1002      +37     

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