@Override
  public synchronized void clear() {
    try {
      mDiskCache.delete();
    } catch (IOException e) {
      Log.d("TileView", "failed to delete disk cache: " + e.getMessage());
    }
  }
Note this is different from MemoryCache.clear, which simply emptie the in-memory map.

Since, presumably, disk-level caching is meant to be persistent, only call this when you're done with the
DiskCache, as it will nt only delete all the file content, but also close (and therefore make inaccessible)
the cache itself.  This might be appriate for a `finish` event (altough most likely not), or perhaps a
"delete cache" or "delete app contents" setting or preference UI.
tileview/src/main/java/com/moagrius/tileview/DiskCache.java