Add runtime type checking of XPath evaluation routines

[janssenhenning]

Added

• eval_xpath_all returns all results
• eval_xpath_one returns one result and makes sure there is only one
• eval_xpath_first returns the first result

All three accept the argument expected_type, which is used to check the results of the XPath evaluation

[codecov]

Codecov Report

Merging #181 (ba7ceb3) into develop (487cf41) will increase coverage by 0.02%.
The diff coverage is 92.72%.

❗ Current head ba7ceb3 differs from pull request most recent head d2e7aaf. Consider uploading reports for the commit d2e7aaf to get more accurate results

@@             Coverage Diff             @@
##           develop     #181      +/-   ##
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+ Coverage    86.56%   86.58%   +0.02%     
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  Files          135      135              
  Lines        24287    24368      +81     
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+ Hits         21025    21100      +75     
- Misses        3262     3268       +6