MAINT: Hide nan warnings for masked internal MA computations

[ahaldane]

This prevents RuntimeWarnings when there is a np.nan value in a masked position in a masked array, eg

>>> a = np.ma.array([0.5, np.nan], mask=[0,1])
>>> a > 0
/usr/bin/ipython2:1: RuntimeWarning: invalid value encountered in greater
#!/usr/bin/python2
masked_array(data = [True --],
             mask = [False  True],
       fill_value = True)

(This PR was split off from #5706)

On my computer I'm still having trouble getting the unit tests actually detect the warnings, even though I see them in a python shell. I'm adding a temporary commit that should cause unit tests to fail, to see if appveyor catches it.

[charris]

"this should fail" is pretty uninformative, needs improvement.

[ahaldane]

The last commit is temporary, I'll remove it in a minute. It reverts some of the changes of the previous commit to make sure the unit tests fail.

[charris]

This change seems radical. What is the intent?

[ahaldane]

The call to filled has no effect since da and db have no masked values (they are just the data arrays of a and b).

[charris]

Some comments would help keep/make the code more understandable.

[mhvk]

Looks good!

[charris]

Thanks Allan.