Add RadfordImpurityDensity model to the SSD extension

[fhagemann]

Follow-up of #72 (got closed by accident when resolving the merge conflicts)

[codecov]

Codecov Report

Attention: Patch coverage is 59.61538% with 21 lines in your changes missing coverage. Please review.

Project coverage is 39.67%. Comparing base (8e1da65) to head (76d2825).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
ext/LegendDataManagementSolidStateDetectorsExt.jl	59.61%	21 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #74      +/-   ##
==========================================
+ Coverage   33.95%   39.67%   +5.72%     
==========================================
  Files          26       27       +1     
  Lines        1944     2009      +65     
==========================================
+ Hits          660      797     +137     
+ Misses       1284     1212      -72     

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[fhagemann]

This PR does, by default, not read the crystal metadata for the impurity density.
However, the default value is increased from -1e9cm^-3 to -5e9cm^-3.

[fhagemann]

Last changes (but should be good to go now):

• Changed the default impurity density to -1e10cm^-3
• Apply the correct order of magnitude (and sign for p-type) when reading the impurity density data
• Add tests for the impurity density read-in

[fhagemann]

@oschulz wants to remove the LsqFit dependency by including relevant fitting code into this package.

[fhagemann]

I would also change the default impurity density value (when no crystal metadata is available) to -0.9e10/cm^3 to match the default value used for fieldgen in LegendGeSim.