Add cuFFT code to spectral solver

[RemiLehe]

While PR #95 implements the spectral solver, it does not link the code to cuFFT. It would be great to add the corresponding code for cuFFT.

Here are a few resources that may help:

• Compiling the code with the spectral solver + GPU:
  This link explains how to compile the code with the spectral solver (on CPU).
  The documentation also explains how to compile the code for GPU on a local machine and on Summit
  Currently, there is of course no documentation on how to do both (spectral+GPU), since WarpX is not currently able to do so. One would probably have to add a variable CUFFT_HOME in Make.WarpX, similar to FFTW_HOME for CPU.

• Implementation: There are placeholders for cuFFT code in PR Implement PSATD solver in C++ #95, in Source/FieldSolver/SpectralSovler/SpectralFieldData.cpp

• Testing the code: One possibility is to use the files inputs.multi.rt (to test the 3D implementation) and inputs.multi.2d.rt (to test the 2D implementation), in Examples/Tests/Langmuir:

./warpx3Dexecutable inputs.multi.rt
./langmuir_multi_analysis.py diags/plotfiles/plt00040

./warpx2Dexecutable inputs.multi.2d.rt
./langmuir_multi_2d_analysis.py diags/plotfiles/plt00040

where warpxXXexecutable should be replaced by the proper executable name (in the Bin folder). In each case, the second line checks the validity of the results (the Python script will raise an AssertionError if the results are not physically valid.)