def __init__(self, name, role, group_by_key=False, units=None, length=None):
super(VectorColumn, self).__init__(name=name,<EOL>role=role,<EOL>group_by_key=group_by_key,<EOL>units=units)<EOL>self.length = length<EOL>
Constructor.

:param name: The name of the column
:type name: str
:param role: The role the column will play in machine learning:
               "Input"
               "Output"
               "Latent Variable"
               "Ignore"
:type role: str
:param group_by_key: Whether or not this column should be used for
    grouping during cross validation
:type group_by_key: bool
:param units: Optionally, the units for this column
:type units: str
:param length: The length of vectors in this column
:type length: int