Modification of spectral fields in IFFT

[tb6g16]

Turns out that one (of possibly more) errors with the local residual was the modification of the spectral field during the IFFT.

To remedy I have added a couple methods with an extra auxiliary spectral field that can be modified during the transform without problem. This obviously isn't particularly desirable since it necessitates an extra spectral array. The ultimate solution will probably come from digging through the docs in extreme detail (I've had a cursory look already and found nothing).

Fields.jl/src/fft.jl

Lines 92 to 102 in 0741e73

	function (f::IFFTPlan!{Ny, Nz, Nt})(u::PhysicalField{Ny, Nz, Nt},
	û::SpectralField{Ny, Nz, Nt},
	û_tmp::SpectralField{Ny, Nz, Nt}) where {Ny, Nz, Nt}
	# copy spectral contents to temporary field
	û_tmp .= û
	# perform transform
	FFTW.unsafe_execute!(f.plan, parent(û_tmp), parent(u))
	return u
	end

[gasagna]

As referenced in #11 (which can be closed), it's fine to have this method. Only use it where necessary.