cdm/src/main/java/ucar/nc2/util/DiskCache.java
static public File getCacheFile(String fileLocation) {
    File f = new File(makeCachePath(fileLocation));
    if (f.exists()) {
      if (!f.setLastModified(System.currentTimeMillis()))
        logger.warn("Failed to setLastModified on " + f.getPath());
    }

    if (!checkExist) {
      File dir = f.getParentFile();
      if (!dir.exists() && !dir.mkdirs())
        logger.warn("Failed to mkdirs on " + dir.getPath());
      checkExist = true;
    }
    return f;
  }
static public File getCacheFile(String fileLocation) {
    File f = new File(makeCachePath(fileLocation));
    if (f.exists()) {
      if (!f.setLastModified(System.currentTimeMillis()))
        logger.warn("Failed to setLastModified on " + f.getPath());
    }

    if (!checkExist) {
      File dir = f.getParentFile();
      if (!dir.exists() && !dir.mkdirs())
        logger.warn("Failed to mkdirs on " + dir.getPath());
      checkExist = true;
    }
    return f;
  }
Get a file in the cache.
File may or may not exist. We assume its always writeable.
If it does exist, set its LastModifiedDate to current time.

@param fileLocation normal file location
@return equivalent File in the cache.