MAINT: Refactor model param parsing for trees, and use concrete cogent3 types

[rmcar17]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 18581689083

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 112 of 116 (96.55%) changed or added relevant lines in 8 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 99.281%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/piqtree/iqtree/_parse_tree_parameters.py	73	77	94.81%

Totals
Change from base Build 18578978507:	0.003%
Covered Lines:	1933
Relevant Lines:	1947

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💛 - Coveralls