Add unit tests

[apizzuto]

• Include seeds for reproducible results

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Add unit tests for:

• Total number conservation (not including secondaries)
• Check that we don't make more energy
• Check absorption matches exponential expectation
• Check earth has right mass
• Check density sampling against a table
• Check CC/NC interaction ratio
• TauDecay checks
• Power-law checks (secondaries coming out comparing against a spline that we know is right)

[apizzuto]

Shell was added with latest PR. We will need to add more tests once other branches are merged

[apizzuto]

Done with most of these. Checks not passing include "not making more energy"
Still need to add earth mass check and some GZK and power law checks