Variable. The name of the Python module and
class containing the variable implementation must be the same as the
un-qualified part of the Opus variable name, e.g. class population is
implemented in file population.py for variable
urbansim.gridcell.population. The module should be stored in a
directory whose name is the dataset name for which the variable is computed,
in the corresponding package. In the above example, population.py is
stored in the gridcell directory in the top level of the
urbansim package directory. This scheme allows Opus to find that
variable. It also means there may be only one Opus variable per Python
variable module.