Remove exec magic from sasdata.quantities.units

[rprospero]

My previous work on refactoring out the units module depended heavily on writing a module at run time and then running the code through exec. While this removed the need to keep _build_tables.py and units.py in sync, it was also very ugly code that could be very frightening to edit.

This PR produces a modified version that edits an exiting units module instead of trying to write a new one from scratch.

• cleaner and easier to read than old code
  • No use of StringIO
  • No use of exec
  • Easier to follow data paths (e.g. the fid.write(f" '{name}': {name}") pattern obscures the actual value in the dictionary).
• units module code isn't run if units module isn't called (the exec version would build the units module the moment someone imported sasdata.quantities instead of sasdata.quantities.units)

Again providing guidance:

• Everything before line 899 of units.py was just moved directly from __init__.py
• The variable name this was chosen to match convention from the Javascript community. I had also considered using self, which is more Pythonic, but had the potential to be confusing, since it is usually attached to a class method.
• The remaining code is still a modification of units.py. However, values are defined directly and moved into the module through setattr.

[codescene-delta-analysis]

No quality gates enabled for this code.

See analysis details in CodeScene

Quality Gate Profile: Custom Configuration
Install CodeScene MCP: safeguard and uplift AI-generated code. Catch issues early with our IDE extension and CLI tool.

[DrPaulSharp]

This all looks good. I have one question, you noted that "Everything before line 899 of units.py was just moved directly from __init__.py". There is also code there that is copied from _units_base.py - did you intend to move that code, or is the copy correct?