Speedup Ewald::density_fft
Speedup Ewald::density_fft

Simple change to avoid multiple access to a mutable reference and
instead use a stack value.
Improves the performance of density_fft by 40%.

Luthaf/cymbalum,g-bauer/lumol,Luthaf/cymbalum,g-bauer/lumol,g-bauer/lumol