NumPy >= 1.4.0 NaN-handling issues

[wesm]

There are a few potential NaN-casting problems floating around the codebase, e.g.:

df1 = pandas.DataFrame({'x':[5]})
df2 = pandas.DataFrame({'x':[1]})
df1.combineAdd(df2)

[wesm]

I believe I've got these all fixed.