cdm/src/main/java/ucar/nc2/ncml/Aggregation.java
protected void setDatasetAcquireProxy(Dataset typicalDataset, NetcdfDataset newds) throws IOException {
    DatasetProxyReader proxy = new DatasetProxyReader(typicalDataset);
    setDatasetAcquireProxy( proxy, newds.getRootGroup());
  }
protected void setDatasetAcquireProxy(Dataset typicalDataset, NetcdfDataset newds) throws IOException {
    DatasetProxyReader proxy = new DatasetProxyReader(typicalDataset);
    setDatasetAcquireProxy( proxy, newds.getRootGroup());
  }
All non-agg variables use a proxy to acquire the file before reading.
If the variable is caching, read data into cache now.
If not caching, VariableEnhanced.setProxyReader() is called.

@param typicalDataset read from a "typical dataset"
@param newds          containing dataset
@throws IOException on i/o error