Increasing test coverage and benchmark bug fix

[joshwlambert]

This PR has:

• Increased the test coverage from the last version release by writing tests for functions that were previously lacking them and adding some more testing to other functions that had incomplete coverage.
• Fixed a bug in benchmark(), in which newly simulated phylogenies were not being used.
• Changed lifecycle badge to stable as the API of the package is now unlikely to vastly change.
• Added some missing fields to DESCRIPTION file.
• Changed plot_phylod() tip labels to show a space and not underscore separated names.

[codecov-commenter]

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