support axis=None for nanmedian ( issue #7352 )

This fixes issue #7352, where ``nanmedian`` does not work when ``axis==None``.
compare the real and imag pars separately and then it works correctly. side issue is to patch ``tm.assert_almost_equal`` to deal with complex numbers by this method

```
In [1]: a=np.array([1+.1111111111111111*1j])

In [2]: b=np.array([1+.1111111111111112*1j])

In [4]: tm.assert_almost_equal(a.real, b.real)
Out[4]: True

In [5]: tm.assert_almost_equal(a.imag, b.imag)
Out[5]: True
```