Convert outgoing netCDF arrays to big-endian

Changeset 76a983ec625 removed the explicit conversion of all outgoing
datatypes into big-endian format (e.g. see page 10 of
http://earthdata.nasa.gov/sites/default/files/esdswg/spg/rfc/esds-rfc-011/ESDS-RFC-011v1.00.pdf)

This changeset converts the type to big-endian during the array
creation in createVariable by calling the dtype.newbyteorder
function with the argument "B" for Big endian.
TST: io: add regression test for netcdf fix