Add toplevel_mutable

[cgarciae]

Changes

• Adds the toplevel_mutable decorator and Immutable.toplevel_mutable method.
• Adds tests for toplevel_mutable

[codecov-commenter]

Codecov Report

Merging #18 (00d6753) into master (e7d0109) will increase coverage by 0.11%.
The diff coverage is 96.66%.

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
+ Coverage   91.70%   91.81%   +0.11%     
==========================================
  Files           6        6              
  Lines         747      770      +23     
==========================================
+ Hits          685      707      +22     
- Misses         62       63       +1     

Impacted Files	Coverage Δ
treeo/__init__.py	100.00% <ø> (ø)
treeo/api.py	91.88% <95.00%> (+0.29%)	⬆️
treeo/mixins.py	87.05% <100.00%> (+0.47%)	⬆️
treeo/tree.py	95.40% <100.00%> (ø)

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