Improve _apply_plan type stability

[niklasschmitz]

The pattern T = ForwardDiff.tagtype(eltype(x)) caused type instabilities in the Dual construction after, so I replace it by an explicit static type parameter declaration @antoine-levitt

Before:

julia> @btime compute_stresses(scfres)
  249.188 ms (2620837 allocations: 246.87 MiB)
3×3 Matrix{Float64}:
 -0.00122464   -2.29076e-18   1.19015e-17
 -2.29076e-18  -0.00122464    5.35591e-18
  8.12389e-18   8.93639e-19  -0.00122464

After:

julia> @btime compute_stresses(scfres)
  66.461 ms (28773 allocations: 112.10 MiB)
3×3 Matrix{Float64}:
 -0.00122464   -2.29076e-18   1.19015e-17
 -2.29076e-18  -0.00122464    5.35591e-18
  8.12389e-18   8.93639e-19  -0.00122464