fix: BasicBlockExits should not be OpTag::DataflowParent

[aborgna-q]

The optags for basic blocks where defined as

graph TD
  classDef transparent fill:#0000,stroke:#0000
  a[...]:::transparent-->DataflowParent
  b[...]:::transparent-->ControlFlowChild
  DataflowParent --> BasicBlock
  ControlFlowChild --> BasicBlock
  BasicBlock --> BasicBlockExit

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notice that BasicBlockExit was a descendant from DataflowParent. This caused errors on code that checked for the tag, such as the force_order pass.

This PR changes the hierarchy to

graph TD
  classDef transparent fill:#0000,stroke:#0000
  a[...]:::transparent-->DataflowParent
  b[...]:::transparent-->ControlFlowChild
  DataflowParent --> DataflowBlock
  ControlFlowChild --> BasicBlockExit
  ControlFlowChild --> DataflowBlock

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Closes #1408

BREAKING CHANGE: Removed OpTag::BasicBlock, replaced with OpTag::DataflowBlock and OpTag::ControlFlowChild

[ss2165]

what is the difference between OpTag::BasicBlock and OpTag::ControlFlowChild now? If none, less noise to just keep BasicBlock but use it how you have used DataflowBlock in this PR, updating the docstring.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.65%. Comparing base (3609736) to head (d5d62a9).
Report is 2 commits behind head on main.

Files	Patch %	Lines
hugr-core/src/ops/tag.rs	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1409      +/-   ##
==========================================
+ Coverage   87.64%   87.65%   +0.01%     
==========================================
  Files         118      118              
  Lines       20358    20394      +36     
  Branches    18040    18042       +2     
==========================================
+ Hits        17843    17877      +34     
- Misses       1724     1726       +2     
  Partials      791      791              

Flag	Coverage Δ
python	91.75% <ø> (+0.03%)	⬆️
rust	87.12% <91.66%> (+<0.01%)	⬆️

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[aborgna-q]

I prefer keeping the DataflowBlock name, since it distinguishes it from BasicBlockExit (otherwise it may be expected that -Exit be a child of BasicBlock).

I'll merge BasicBlock and ControlFlowChild

[ss2165]

LGTM