Implement isfinite, isinf, isnan ufuncs

[juntyr]

Based on and blocked on #118, only the last commit is new

Progress towards #111

[juntyr]

I get some weird runtime warnings in the isfinite and isinf ufuncs:

tests/test_quaddtype.py: 10 warnings
  /workspaces/numpy-user-dtypes/quaddtype/tests/test_quaddtype.py:113: RuntimeWarning: invalid value encountered in isfinite
    quad_result = op_func(quad_val)

tests/test_quaddtype.py: 10 warnings
  /workspaces/numpy-user-dtypes/quaddtype/tests/test_quaddtype.py:113: RuntimeWarning: invalid value encountered in isinf
    quad_result = op_func(quad_val)

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

Any idea what I'm doing wrong?

[seberg]

IEEE says comparisons with NaNs trigger FPEs (floating point error; FPEs are a flag set on the CPU when one occurs basically). That means normally also nan == inf.
C99 has special comparison functions for this purpose (not that all compilers used to get it right). Maybe sleef gets these warnings right, maybe not. But the best solution would be to avoid the function triggering it. Maybe sleef has a quiet comparison or just a way to test for this without a comparison.