v0.6.0: Complex numbers + FFT + MacOS build (?)

[janpfeifer]

Changes:

• FFT, RealFFT, InverseFFT and InverseRealFFT operations.
  • Added a small notebook demo for FFT.
• Added Complex/Imag/Real/Conj operations to manipulate complex numbers (and their gradients).
• Added support for complex numbers for ConvertType. Defined gradient for ConvertType.
• Added Complex128 and Complex64 dtypes support.
• Added "spacers" (like "*" for axis ranges) and AxisElem() for Slice().
• Package examples/notebook/gonb/margaid: Added Plots.AddValues and Plots.PlotToHTML;
  Fixed Plots returned by New() to be linear scale by default.
• Included build of tcmalloc (gperftools) from the c/ directory, when building libgomlx_xla.so.
  Still the libtcmalloc.so is needed in runtime.
  A copy is included in the gomlx_xla.tar.gz package (under lib/gomlx) and can be copied from there if needed.
  This enables build for Macs — see GoMLX+XLA C library does not compile on MacOS #23.

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This discussion was created from the release v0.6.0: Complex numbers + FFT + MacOS build (?).