These warning filters are not threadsafe, while it is nicer if the
comparison functions can be called in a threaded environment since
they do no explicite warning checking.

Less filters are also cleaner in general, though may also mean that
downstream projects see warnings that were previously hidden.

These now delegate to __getitem__, so subclasses no longer need to define them

This code was only here for Python 2.5 compatibility, but numpy requires 2.7
at minimum

gh-8410 breaks a large number of astropy tests, because it sets up
a boolean array for values that should actually be compared (i.e.,
are not `nan` or `inf`) using `zeros_like`. The latter means that
for subclasses, the boolean test array is not a plain `ndarray` but
the subclass. But for astropy's `Quantity`, the `all` method is
undefined.

This commit ensures the test arrays from `isinf` and `isnan` are
used directly.

See issue #2418

assert_array_less does now treat inf/nan
correctly, namely:
-inf < x < inf for any real number x
nans still are not compared, when they occur
at the same position

To achieve this without breaking any other
functions relying on assert_array_compare,
an additional parameter (equal_inf) was
introduced that defaults to the old behavior
(analogous to equal_nan).

This failed to raise AssertionError in an earlier rewrite of
assert_array_compare and is thus tested explicitly now.

Added tests for simple arrays/elements,
tests that ensure that nans are not compared when they are in the same
position in both arrays but raise AssertionError when they are not,
and tests that ensure inf logic
(-np.inf < np.inf, 1.0 < np.inf, -np.inf < 1.0)
and that all other cases fail.