MIPField: Fix for correct mapping in loadLevelFromDisk().
          Mutex on HDF5 calls in LazyLoadAction.
          Fix for MIPField<SparseField> to work with sparse cache.

SparseField: Fix for LinearInterp to hold reference to block while accessing data.
             Global lock on Block::resize(). Prevents need for per-block mutex.
             Added isDynamicLoad(), incBlockRef(), activateBlock(), decBlockRef().
             memSize() will no longer count deactivated blocks.