Fix reltol type instability in Newton solver

The issue was that reltol could be either a scalar (when adaptive=false)
or the value from integrator.opts.reltol (which could be scalar or vector).
This runtime branch caused type instability.

The fix checks both conditions at once to ensure reltol is only set to
eps(eltype(dz)) when it would actually be a scalar, avoiding the type
instability when reltol is a vector.

Fixes part of issue #2819

Author: ChrisRackauckas

lib/OrdinaryDiffEqNonlinearSolve/src/newton.jl

@@ -246,10 +246,10 @@ end
         update_coefficients!(W, ustep, p, tstep; dtgamma = γW, transform = true)
     end
 
-    if integrator.opts.adaptive
-        reltol = integrator.opts.reltol
-    else
+    if !integrator.opts.adaptive && !(integrator.opts.reltol isa AbstractArray)
         reltol = eps(eltype(dz))
+    else
+        reltol = integrator.opts.reltol
     end
 
     if is_always_new(nlsolver) || (iter == 1 && new_W)