Provide a more helpful error message for constructor

Author: bjack205

docs/src/index.md

@@ -52,9 +52,9 @@ A specific version can be specified using
 ```
 (v1.5) pkg> add [email protected]
 ```
-Or you can check out the master branch with
+Or you can check out the main branch with
 ```
-(v1.5) pkg> add TrajectoryOptimization#master
+(v1.5) pkg> add TrajectoryOptimization#main
 ```
 Lastly, if you want to clone the repo into your `.julia/dev/` directory for development, you can use
 ```

src/problem.jl

@@ -84,6 +84,12 @@ function Problem(models::Vector{<:DiscreteDynamics}, obj::O, x0::AbstractVector,
         U0=[fill(0.0, RD.control_dim(model)) for model in models],
         kwargs...) where {O}
 
+    if :x0 in keys(kwargs)
+        throw(ArgumentError("Cannot pass x0 as a keyword argument. It is now a positional argument, and xf is a keyword argument.\n" * 
+            "\tUse Problem(model, obj, x0, tf, xf=xf, kwargs...) instead."
+        ))
+    end
+
     # Check control dimensions
     nx,nu = RD.dims(models)
     same_state_dimension = all(x->x == nx[1], nx)

test/problems_tests.jl

@@ -213,4 +213,6 @@ rollout!(prob)
 
 @test RD.setinitialtime!(prob, 1.0) == 1.0 + tf
 @test prob.Z[1].t == 1.0
-@test prob.Z[end].t == 1.0 + tf
+@test prob.Z[end].t == 1.0 + tf
+
+@test_throws ArgumentError Problem(dmodel, obj, xf, tf, x0=x0)