Provide Base.oneunit()

```
julia> euclidean(a, b)
ERROR: MethodError: no method matching VariableRef(::AffExpr)

Closest candidates are:
  (::Type{GenericVariableRef{T}} where T)(::Any, ::Any)
   @ JuMP ~/.julia/packages/JuMP/h0lrf/src/variables.jl:251
  GenericVariableRef{T}(::ConstraintRef{GenericModel{T}, <:MathOptInterface.ConstraintIndex{MathOptInterface.VariableIndex}}) where T
   @ JuMP ~/.julia/packages/JuMP/h0lrf/src/variables.jl:520
  GenericVariableRef{T}(::GenericModel{T}) where T
   @ JuMP ~/.julia/packages/JuMP/h0lrf/src/variables.jl:494

Stacktrace:
 [1] oneunit(#unused#::Type{VariableRef})
   @ Base ./number.jl:370
 [2] _eval_start(d::Euclidean, #unused#::Type{VariableRef}, #unused#::Type{VariableRef}, #unused#::Nothing)
   @ Distances ~/.julia/packages/Distances/PvoXa/src/metrics.jl:320
 [3] _eval_start(d::Euclidean, #unused#::Type{VariableRef}, #unused#::Type{VariableRef})
   @ Distances ~/.julia/packages/Distances/PvoXa/src/metrics.jl:318
 [4] eval_start(d::Euclidean, a::VariableRef, b::VariableRef)
   @ Distances ~/.julia/packages/Distances/PvoXa/src/metrics.jl:317
 [5] _evaluate(d::Euclidean, a::VariableRef, b::VariableRef, #unused#::Nothing)
   @ Distances ~/.julia/packages/Distances/PvoXa/src/metrics.jl:236
 [6] (::Euclidean)(a::VariableRef, b::VariableRef)
   @ Distances ~/.julia/packages/Distances/PvoXa/src/metrics.jl:328
 [7] top-level scope
   @ REPL[8]:1

```

src/aff_expr.jl

@@ -203,8 +203,12 @@ Base.zero(a::GenericAffExpr) = zero(typeof(a))
 function Base.one(::Type{GenericAffExpr{C,V}}) where {C,V}
     return GenericAffExpr{C,V}(one(C), OrderedDict{V,C}())
 end
+function Base.oneunit(::Type{GenericAffExpr{C,V}}) where {C,V}
+    return GenericAffExpr{C,V}(oneunit(C), OrderedDict{V,C}())
+end
 
 Base.one(a::GenericAffExpr) = one(typeof(a))
+Base.oneunit(a::GenericAffExpr) = oneunit(typeof(a))
 
 Base.copy(a::GenericAffExpr) = GenericAffExpr(copy(a.constant), copy(a.terms))
 

src/variables.jl

@@ -360,10 +360,14 @@ function Base.zero(::Type{V}) where {V<:AbstractVariableRef}
 end
 
 Base.one(v::AbstractVariableRef) = one(typeof(v))
+Base.oneunit(v::AbstractVariableRef) = oneunit(typeof(v))
 
 function Base.one(::Type{V}) where {V<:AbstractVariableRef}
     return one(GenericAffExpr{value_type(V),V})
 end
+function Base.oneunit(::Type{V}) where {V<:AbstractVariableRef}
+    return oneunit(GenericAffExpr{value_type(V),V})
+end
 
 """
     coefficient(v1::GenericVariableRef{T}, v2::GenericVariableRef{T}) where {T}