MAINT: re-eanble float32 computations through fused types.

This is not faster, need to reconsider later whether this is actually useful.
MAINT: re-enable float32 for upcoef(), downcoef() and swt().
MAINT: make downcoef() accept array_like input.
Merge branch 'enable-float32' into master.

Closes gh-35
Closes gh-9