Tests if this model can be modified. Modification
of a model that is not editable will result
in CoreException being thrown.
true if this model can be modified
Tests if this model valid. When models
are loaded from the file, they may pass the
syntax error checking and load all the model objects.
However, some of the objects may contain invalid
values that make the model unusable.
true only if the model can be safely used in allcomputations.