Fix particle source array to be correct length n_source

[jcurtis2]

The length of the particle source array was set incorrectly and the test failed to pick this up as the correct value and incorrect size were both 1. Since the test didn't trigger the problem, I added multiple sources to the test. Test asserts that particle source array length is equal the number of sources (now 2), array type is int, and that each particle source array sums to 1, which is true the absence of coagulation.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.89%. Comparing base (16fed1f) to head (5ceb834).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #545      +/-   ##
==========================================
+ Coverage   91.81%   91.89%   +0.07%     
==========================================
  Files          56       56              
  Lines        2530     2530              
  Branches      144      144              
==========================================
+ Hits         2323     2325       +2     
+ Misses        132      131       -1     
+ Partials       75       74       -1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.