Adding support for .spz files

[Joel-Y-18]

Small PR to add support for .spz outputs. Closes #164. In the issue there was a suggestion to wait until it is clear that .spz will be supported in the future - this seems to be the case.

I am a new contributor, so feedback is welcome.

Outputs are identical (300 iterations on CPU):

[pierotofy]

Thanks @Joel-Y-18 this looks great ! 🙏

Before merging we'll just have to wait for nianticlabs/spz#89 and a new release of spz because it looks like they pulled zlib 1.3.1 (which is a dep of spz) and it currently breaks the build on Windows.

  -- Using src='https://zlib.net/zlib-1.3.1.tar.gz'
  CMake Error at zlib-subbuild/zlib-populate-prefix/src/zlib-populate-stamp/download-zlib-populate.cmake:163 (message):
    Each download failed!

Another option could be to make .spz support optional via a cmake flag.