Fix num2date / date2num multidimensional arrays. Remove iterator support. Fix masked arrays

[djkirkham]

Fixes a bug where these functions wouldn't work with multidimensional arrays following a recent change.

I've also removed support for passing iterators to these functions to keep the code clean. Iterators weren't supported before the previous change.

[coveralls]

Coverage decreased (-0.01%) to 90.526% when pulling 3f3301d on djkirkham:num2date-rounding into 3c7ee1b on SciTools:master.

[djkirkham]

Previously the num2date functions would return None in an maked array where the input was masked. The date2num function would ignore the mask. I have maintained this functionality

[coveralls]

Coverage decreased (-0.3%) to 90.226% when pulling e6f6ab1 on djkirkham:num2date-rounding into 3c7ee1b on SciTools:master.

[coveralls]

Coverage decreased (-0.01%) to 90.526% when pulling 3bc6d71 on djkirkham:num2date-rounding into 3c7ee1b on SciTools:master.