Revert to :slow as the default matmul mode

Author: OlivierHnt

src/IntervalArithmetic.jl

@@ -210,7 +210,7 @@ Configure the default behavior for:
   definition of matrix multiplication. Learn more:
   [`IntervalArithmetic.MatMulMode`](@ref).
 """
-function configure(; numtype::Type{<:NumTypes}=Float64, flavor::Symbol=:set_based, rounding::Symbol=:correct, power::Symbol=:fast, matmul::Symbol=:fast)
+function configure(; numtype::Type{<:NumTypes}=Float64, flavor::Symbol=:set_based, rounding::Symbol=:correct, power::Symbol=:fast, matmul::Symbol=:slow)
     configure_numtype(numtype)
     configure_flavor(flavor)
     configure_rounding(rounding)