2021.1b6 - first PyPI version

First beta version supporting:

• CUDA and OpenCL backends
• C2C, R2C/C2R for inplace and out-of-place transforms
• single and double precision for all transforms
• 1D, 2D and 3D transforms (always performed from the last axes).
• 1D and 2D FFT accept arrays of any number of dimensions (batch transforms), 3D FFT
  only accepts 3D arrays.
• allowed prime factors (radix) of the transform axes are 2, 3, 5, 7, 11 and 13
• normalisation=0 (array L2 norm * array size on each transform) and 1 (the backward
  transform divides the L2 norm by the array size, so FFT*iFFT restores the original array)
• now testing the FFT size does not exceed the allowed maximum prime number decomposition (13)
• unit tests for all transforms: see test sub-directory.
• Note that out-of-place C2R transform currently destroys the complex array for FFT dimensions >=2
• tested on macOS (10.13.6) and Linux.
• inplace transforms do not require an extra buffer or work area (as in cuFFT), unless the x
  size is larger than 8192, or if the y and z FFT size are larger than 2048. In that case
  a buffer of a size equal to the array is necessary. This makes larger FFT transforms possible
  based on memory requiremnts (even for R2C !) compared to cuFFT. For example you can compute
  the 3D FFT for a 1600**3 complex64 array withon 32GB of memory.