Tweaks to wgridder and nufft components

See merge request !177

Guard use of mutex, cv, lock when DUCC0_NO_THREADING is defined.

For TF, we need them completely disabled.  They are disabled
in `threading.cc` and `fft.h`, so this seems consistent.

Tweak space filling curves

See merge request !175

Switch Julia wrapper to expect one-based indices etc.

See merge request !176

Julia SHT support

See merge request !173