Support reality symmetry (hermitian symmetry)

[CosmoMatt]

This PR adds support for real signals, e.g. Hermitian symmetry. This includes adding support for

• Real forward transforms
  • mw
  • mwss
  • dh
  • healpix
• Real inverse transforms
  • mw
  • mwss
  • dh
  • healpix

Currently this PR updates transforms to accelerate evaluation but does not change memory overhead. A further factor of 2 saving in memory could be achieved at the cost of slightly more opaque indexing to arrays. Finally, adding support for real signals within single transforms is getting somewhat cluttered, but perhaps this is ok?

[CosmoMatt]

@jasonmcewen currently reality is also supported for healpix sampling, but given the extra caveats around the healpix_ffts switching from complex to real FFTs is probably not worth chopping up the code. Especially as we're doing SOV so the compute time, which is additive, is dominated by the primary loop not the FFTs.

If you think that's ok then we can tick off healpix support and get this merged.

[jasonmcewen]

Looks great @CosmoMatt, as discussed. Let's merge this and consider Healpix in another PR.