Unit tests core components

[esteinig]

Implement appropriate unit tests for paf.rs and covplot.rs

Excluded for now:

• CovPlot::to_console() - not sure how to implement tests for crossterm output to stdout
• PafAlignment::to_console() - not sure how to test output tostdout
• PafAlignment::coverage_plots() - same issue with crossterm unit testing

[esteinig]

Should probably format the strings for CovPlot and PafAlignment for testing, then output to console in a separate print function

[esteinig]

This can actually be done nicely in tests/app.rs binary testing