Add SaltXML importer

[thomaskrause]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 79.40828% with 174 lines in your changes missing coverage. Please review.

Project coverage is 82.51%. Comparing base (ac5c4df) to head (d0b87ba).

Files	Patch %	Lines
src/importer/saltxml/document.rs	80.65%	25 Missing and 81 partials ⚠️
src/importer/saltxml.rs	79.86%	15 Missing and 14 partials ⚠️
src/importer/saltxml/corpus_structure.rs	75.24%	15 Missing and 10 partials ⚠️
src/manipulator/visualize.rs	59.09%	8 Missing and 1 partial ⚠️
src/exporter/graphml.rs	82.60%	3 Missing and 1 partial ⚠️
src/lib.rs	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #261      +/-   ##
==========================================
+ Coverage   82.40%   82.51%   +0.11%     
==========================================
  Files          42       45       +3     
  Lines       15588    16406     +818     
  Branches    15588    16406     +818     
==========================================
+ Hits        12845    13538     +693     
- Misses       1034     1052      +18     
- Partials     1709     1816     +107     

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