Make pyfftw and rasterio extra dependencies

[akx]

This will make the installation footprint of the package smaller by default.

• pyFFTW not strictly required; it's just a faster FFT backend (used by Phasepack, hence by the FSIM metric).
  • In addition, since FFTW is licensed under the GPL, a strict dependency on pyFFTW (a GPL wrapper) would also make any application using it require GPL (or purchasing a license for FFTW). See License is misleading; programs using pyFFTW still need to comply with the GPL pyFFTW/pyFFTW#229 for discussion. Making it a soft dependency allows this library to remain neatly MIT.
• rasterio is only needed for reading TIFFs in an... alternate way, it seems? I guess it has to do with some multi-channel TIFFs? Either way, it's not required for using the package as a library (which I'm doing, for instance).

This also expands the GHA workflow to:

• test all three out of four of these configurations (no extras, speedups, speedups + rasterio).
• test all the metrics

[nekhtiari]

LGTM, @markusuwe please have a look!