def to_mongo(self, value, **kwargs):
        return self.to_python(value)
parse self.__value into a dict{kper:Util2d}
dataset/ETHPy150Open MongoEngine/mongoengine/mongoengine/base/fields.py/BaseField.to_mongo--crossover with--dataset/ETHPy150Open modflowpy/flopy/flopy/utils/util_array.py/Transient2d.build_transient_sequence