Split tree functionality into standalone utility

[Pennycook]

Feature/behavior summary

The functionality that is currently exposed by -R files should be exposed as a dedicated cbi-tree utility instead.

Request attributes

• Would this be a refactor of existing code?
• Does this proposal require new package dependencies?
• Would this change break backwards compatibility?

Related issues

Initially proposed by @laserkelvin in #147.

Solution description

• Remove the -R files option.
• Add a new cbi-tree utility accepting similar options to codebasin (e.g., -x, analysis.toml).
• Rename cbicov to cbi-cov (for consistency with cbi-tree, which is easier to read than cbitree).

Additional notes

Making cbi-tree into a standalone tool opens up some interesting possibilities for future work, including:

• Adjusting the default output (e.g., to suppress warnings);
• Adding new, tree-specific, command-line options.