cdm/src/main/java/ucar/nc2/dataset/DatasetConstructor.java
static public void transferDataset(NetcdfFile src, NetcdfDataset target, ReplaceVariableCheck replaceCheck) {
    transferGroup(src, target, src.getRootGroup(), target.getRootGroup(), replaceCheck);
  }
static public void transferDataset(NetcdfFile src, NetcdfDataset target, ReplaceVariableCheck replaceCheck) {
    transferGroup(src, target, src.getRootGroup(), target.getRootGroup(), replaceCheck);
  }
Copy contents of "src" to "target". skip ones that already exist (by name).
Dimensions and Variables are replaced with equivalent elements, but unlimited dimensions are turned into regular dimensions.
Attribute doesnt have to be replaced because its immutable, so its copied by reference.

@param src transfer from here. If src is a NetcdfDataset, transferred variables get reparented to target group.
@param target transfer to this NetcdfDataset.
@param replaceCheck if null, add if a Variable of the same name doesnt already exist, otherwise
replace if replaceCheck.replace( Variable v) is true