Do not warn when invalid values are masked in a MaskedArray

When doing a mathematical operation on a masked array, and some elements are invalid for the particular mathematical operation, numpy should not issue a warning if all invalid elements are masked.

This cannot be handled by numpy.seterr: I want numpy to warn or raise when performing an invalid operation on any non-masked value, but to be silent when performing an invalid operation on a masked value.

In [1]: A = arange(-2, 5)/2

In [2]: Am = numpy.ma.masked_less_equal(A, 0)

In [3]: numpy.log10(Am)
/export/data/home/gholl/venv/gerrit/bin/ipython3:1: RuntimeWarning: divide by zero encountered in log10
  #!/export/data/home/gholl/venv/gerrit/bin/python3.4
/export/data/home/gholl/venv/gerrit/bin/ipython3:1: RuntimeWarning: invalid value encountered in log10
  #!/export/data/home/gholl/venv/gerrit/bin/python3.4
Out[3]: 
masked_array(data = [-- -- -- -0.3010299956639812 0.0 0.17609125905568124 0.3010299956639812],
             mask = [ True  True  True False False False False],
       fill_value = 1e+20)

[efiring]

The problem also appears in simple comparisons:

In [2]: np.ma.masked_invalid([1.1, np.nan]) > 1
/Users/efiring/anaconda/envs/test/bin/ipython:1: RuntimeWarning: invalid value encountered in greater
  #!/Users/efiring/anaconda/envs/test/bin/python

masked_array(data = [True --],
             mask = [False  True],
       fill_value = True)

[rgommers]

@greglucas that sounds great, a PR would be nice.

This idea was brought up in #15200 suggesting this may be a good thing to do.

Yes, I agree with those comments.