Cleaned up ScorePackageManager._get_tempo_inventory(). MODEL.

TempoInventoryMaterialManager is now deprecated.

MODEL: the goal will be to remove all material package manager classes and
leave only MaterialPackageManager.

Here are the current material package manager classes:

    * MaterialPackageManager
        * ArticulationHandlerMaterialManager
        * DynamicHandlerMaterialManager
        * ListMaterialManager
        * MarkupInventoryMaterialManager
        * OctaveTranspositionMappingInventoryMaterialManager
        * PitchRangeInventoryMaterialManager
        * RhythmMakerMaterialManager
        * SargassoMeasureMaterialManager
        * TempoInventoryMaterialManager