Add support for if-expressions

[AnHeuermann]

Description

Equations containing if-expressions are not supported by the parser.
When trying nested if-expressions (generated from if-elseif-else expressions) the parser get's stuck.

How to reproduce

Example 1: Inline if

InlineIf.mo

package 'InlineIf'
  model 'InlineIf'
    Real 'x';
  equation
    'x' = if time < 0.5 then 1.0 else 2.0;
  end 'InlineIf';
end 'InlineIf';

julia> BaseModelica.parse_basemodelica("InlineIf.mo")
ERROR: MethodError: no method matching -(::Nothing, ::SymbolicUtils.BasicSymbolic{Real})
The function `-` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  -(::ChainRulesCore.NoTangent, ::Any)
   @ ChainRulesCore ~/.julia/packages/ChainRulesCore/Vsbj9/src/tangent_arithmetic.jl:61
  -(::Any, ::ChainRulesCore.ZeroTangent)
   @ ChainRulesCore ~/.julia/packages/ChainRulesCore/Vsbj9/src/tangent_arithmetic.jl:102
  -(::MutableArithmetics.Zero, ::Any)
   @ MutableArithmetics ~/.julia/packages/MutableArithmetics/tNSBd/src/rewrite.jl:67
  ...

Stacktrace:
  [1] ModelingToolkit.TearingState(sys::ModelingToolkit.ODESystem; quick_cancel::Bool, check::Bool)
    @ ModelingToolkit ~/.julia/packages/ModelingToolkit/S1kqK/src/systems/systemstructure.jl:290
  [2] ModelingToolkit.TearingState(sys::ModelingToolkit.ODESystem)
    @ ModelingToolkit ~/.julia/packages/ModelingToolkit/S1kqK/src/systems/systemstructure.jl:256
  [3] __structural_simplify(sys::ModelingToolkit.ODESystem, io::Nothing; simplify::Bool, kwargs::@Kwargs{…})
    @ ModelingToolkit ~/.julia/packages/ModelingToolkit/S1kqK/src/systems/systems.jl:75
  [4] __structural_simplify
    @ ~/.julia/packages/ModelingToolkit/S1kqK/src/systems/systems.jl:72 [inlined]
  [5] structural_simplify(sys::ModelingToolkit.ODESystem, io::Nothing; additional_passes::Vector{…}, simplify::Bool, split::Bool, allow_symbolic::Bool, allow_parameter::Bool, conservative::Bool, fully_determined::Bool, kwargs::@Kwargs{})
    @ ModelingToolkit ~/.julia/packages/ModelingToolkit/S1kqK/src/systems/systems.jl:33
  [6] structural_simplify (repeats 2 times)
    @ ~/.julia/packages/ModelingToolkit/S1kqK/src/systems/systems.jl:28 [inlined]
  [7] eval_AST(model::BaseModelica.BaseModelicaModel)
    @ BaseModelica ~/.julia/packages/BaseModelica/qCG6a/src/evaluator.jl:127
  [8] eval_AST(package::BaseModelica.BaseModelicaPackage)
    @ BaseModelica ~/.julia/packages/BaseModelica/qCG6a/src/evaluator.jl:132
  [9] baseModelica_to_ModelingToolkit
    @ ~/.julia/packages/BaseModelica/qCG6a/src/evaluator.jl:153 [inlined]
 [10] parse_basemodelica(filename::String)
    @ BaseModelica ~/.julia/packages/BaseModelica/qCG6a/src/BaseModelica.jl:23
 [11] top-level scope
    @ REPL[2]:1
Some type information was truncated. Use `show(err)` to see complete types.

Example 2: Nested inline if

package 'InlineIfNested'
  model 'InlineIfNested'
    Real 'x';
  equation
    'x' = if time < 0.33 then 1.0 else if time < 0.66 then 2.0 else 3.0;
  end 'InlineIfNested';
end 'InlineIfNested';

julia> BaseModelica.parse_basemodelica("InlineIfNested.mo")

This never finishes (or at leats I didn't wanted to wate more than a few minutes).

Versions 'n Stuff

• Julia Version 1.11.6
• BaseModelica v1.1.0
• OS: Ubuntu 24.04.3 LTS