Make sure that the Converter class handles masked arrays correctly

[ckhroulev]

See the title.

[aaschwanden]

Constantine,

I've been running into this issue again and realize it's still open. Would it be hard to fix?

Minimal example:

import numpy as np
from udunits2 import Converter, System, Unit 
sys = System()

data = np.array([1000., 1000.])
mask = np.array([0, 1])

x = np.ma.array(data=data, mask=mask)
i_unit = Unit(sys, "m")
o_unit = Unit(sys, "km")

c = Converter((i_unit, o_unit))
y = c(x)
y = [1., 1.]

instead of

y = [1., --]