The order number written to Alembic is the same as we use in memory, so
the +1 wasn't needed, at least according to the reference Maya exporter
maya/AbcExport/MayaNurbsCurveWriter.cpp, function
MayaNurbsCurveWriter::write(), in the Alembic source code.
Furthermore, when writing an array of nurb orders, the curve type should
be set to kVariableOrder, otherwise the importer will ignore it.
This test checks that a set of cubes are exported with the correct
transform, both with flatten=True and flatten=False.
This commit also adds an easy to use superclass for upcoming Alembic
unit tests.
