Fix ifelse support and update ModelingToolkit compat

This PR fixes issue #36 by:

1. Adding an `ifelse` helper function definition in MATLAB
   - MATLAB doesn't have a built-in `ifelse` function, but Symbolics.jl
     generates code using `ifelse(cond, a, b)` for the MATLAB target
   - We now define `ifelse = @(cond, a, b) cond .* a + ~cond .* b;`
     before evaluating the generated MATLAB code

2. Updating ModelingToolkit compat to support versions 8, 9, 10, 11
   - The original error "TypeError: non-boolean (Num) used in boolean context"
     was fixed in newer ModelingToolkit/Symbolics versions
   - Added compatibility shim for the `states` -> `unknowns` API change

Fixes #36

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: ChrisRackauckas

Project.toml

@@ -11,7 +11,7 @@ Reexport = "189a3867-3050-52da-a836-e630ba90ab69"
 [compat]
 DiffEqBase = "6.122"
 MATLAB = "0.8"
-ModelingToolkit = "8"
+ModelingToolkit = "8, 9, 10, 11"
 Reexport = "0.2, 1.0"
 julia = "1.6"
 

src/MATLABDiffEq.jl

@@ -4,6 +4,13 @@ using Reexport
 @reexport using DiffEqBase
 using MATLAB, ModelingToolkit
 
+# Handle ModelingToolkit API changes: states -> unknowns
+if isdefined(ModelingToolkit, :unknowns)
+    const mtk_states = ModelingToolkit.unknowns
+else
+    const mtk_states = ModelingToolkit.states
+end
+
 abstract type MATLABAlgorithm <: DiffEqBase.AbstractODEAlgorithm end
 struct ode23 <: MATLABAlgorithm end
 struct ode45 <: MATLABAlgorithm end
@@ -57,7 +64,7 @@ function DiffEqBase.__solve(
 
     matstr = ModelingToolkit.build_function(
         map(x -> x.rhs, equations(sys)),
-        states(sys),
+        mtk_states(sys),
         parameters(sys),
         independent_variables(sys)[1],
         target = ModelingToolkit.MATLABTarget()
@@ -70,6 +77,11 @@ function DiffEqBase.__solve(
     put_variable(get_default_msession(), :reltol, reltol)
     put_variable(get_default_msession(), :abstol, abstol)
 
+    # Define the ifelse helper function in MATLAB
+    # MATLAB doesn't have a built-in ifelse, so we need to define one
+    # This allows symbolic ifelse expressions to be evaluated properly
+    eval_string("ifelse = @(cond, a, b) cond .* a + ~cond .* b;")
+
     # Send the function over
     eval_string(matstr)