Sigma31 -> sigma13 to match improvement term in Dw!

src/Spinors/Spinors.jl

@@ -326,7 +326,7 @@ indexing for Dirac basis ``\\Gamma_n``:
 12  sigma03
 13  sigma12
 14  sigma23
-15  sigma31
+15  sigma13
 16  identity
 
 """
@@ -344,7 +344,7 @@ indexing for Dirac basis ``\\Gamma_n``:
 @inline dmul(::Type{Gamma{12}}, a::Spinor{NS,G}) where {NS,G} = Spinor{NS,G}(( a.s[1], -a.s[2],  -a.s[3],  a.s[4]))
 @inline dmul(::Type{Gamma{13}}, a::Spinor{NS,G}) where {NS,G} = Spinor{NS,G}((-a.s[1],  a.s[2],  -a.s[3],  a.s[4]))
 @inline dmul(::Type{Gamma{14}}, a::Spinor{NS,G}) where {NS,G} = Spinor{NS,G}((-a.s[2], -a.s[1],  -a.s[4], -a.s[3]))
-@inline dmul(::Type{Gamma{15}}, a::Spinor{NS,G}) where {NS,G} = Spinor{NS,G}(( imm(a.s[2]), mimm(a.s[1]),   imm(a.s[4]), mimm(a.s[3])))
+@inline dmul(::Type{Gamma{15}}, a::Spinor{NS,G}) where {NS,G} = Spinor{NS,G}(( mimm(a.s[2]), imm(a.s[1]),  mimm(a.s[4]), imm(a.s[3])))
 @inline dmul(::Type{Gamma{16}}, a::Spinor{NS,G}) where {NS,G} = a
 
 export Spinor, Pgamma, Gamma