SleefQuadPrecision arrays are missing isnan, isinf, isfinite, sign, copysign ufuncs

[juntyr]

isnan, isinf, isfinite, and sign should be implementable using comparison functions, for copysign sleef contains an implementation.

I'd be happy to submit a PR for these ufuncs if you could point me in the right direction of where to add them.

[ngoldbaum]

Makes sense to me.

Look inside umath.cpp. Here's where StringDType sets up isnan inside NumPy: https://github.com/numpy/numpy/blob/85708b3c0bd3d2fdbcb00cb08d0a7b323ce3af2d/numpy/_core/src/umath/stringdtype_ufuncs.cpp#L2650-L2655

There's an older version of StringDType in this repo too.

[juntyr]

Also big thanks to you and @SwayamInSync for this package - I've been using it for ~half a year now and despite these small inconveniences I've had to patch over it is such a big help!