RFE: sym command "replace true" or "keepOriginal false" option

[Elaine Meng]

Would like option that modifies "copies true" to also close the original model and replace it with the new model. To try a different assembly one would need to reopen the original, but this is not a problem for the most common case of mmCIF fetch. The reason is that the extra hidden model can be confusing and slow down calculations when user doesn't realize to exclude the hidden model from the calculations.