separated read-only container (Container) from modifiable container (OpenContainer)
ExternalValuesContainer. Some renaming of classes for clarity.
removed virtual destructors in the object hierarchy. This is safe so long as the destructors aren't needed (we just need to free the memory occupied by the object.)

Wthout d'tors:
Program Memory Usage 	:	6858 bytes   20.9 % Full
Data Memory Usage 		:	552 bytes   27.0 % Full
With d'tors:
Program Memory Usage 	:	7696 bytes   23.5 % Full
Data Memory Usage 		:	680 bytes   33.2 % Full