Add support for negative variables (unary minus) (#35)

[ChrisRackauckas-Claude]

Summary

This PR adds support for unary minus operations (negative variables) in BaseModelica expressions.

Problem

The parser could not handle negative variables like -'x' in equations such as der('x') = -'x'. The issue was that the create_arithmetic_expression function expected a left operand when encountering a minus operator, but with unary minus there is no left operand.

Solution

1. Added new AST node: BaseModelicaUnaryMinus(operand) to represent unary minus operations
2. Updated parser logic: Modified create_arithmetic_expression to detect when the first element is a minus operator and handle it as unary minus
3. Added evaluation support: Added evaluation logic for BaseModelicaUnaryMinus in the evaluator (-f(operand))

Changes

• src/parser.jl:
  • Added BaseModelicaUnaryMinus(operand) AST node
  • Updated create_arithmetic_expression function to handle unary minus case
• src/evaluator.jl:
  • Added evaluation case for BaseModelicaUnaryMinus(operand) => -f(operand)
• test/runtests.jl:
  • Added unit test for simple unary minus parsing (-5)
  • Added integration test for negative variables in full model
• test/testfiles/NegativeVariable.mo:
  • Added test file with the example from the issue

Test plan

• Added unit test for parsing -5 as BaseModelicaUnaryMinus
• Added integration test parsing the full model from issue Add support for negative variables #35
• Tests verify both parsing and evaluation work correctly
• Existing tests continue to pass

Fixes

Closes #35

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