cdm/src/main/java/thredds/filesystem/MFileOS7.java
static public MFileOS7 getExistingFile(String filename) throws IOException {
    if (filename == null) return null;
    Path path = Paths.get(filename);
    if (Files.exists(path)) return new MFileOS7(path);
    return null;
  }
static public MFileOS7 getExistingFile(String filename) throws IOException {
    if (filename == null) return null;
    Path path = Paths.get(filename);
    if (Files.exists(path)) return new MFileOS7(path);
    return null;
  }
Make MFileOS7 if file exists, otherwise return null
@param filename full path name
@return MFileOS or null