If no obs file is found, return None,None rather than raise an exception.
Return None if things didn't work out (rather than crash).
Derived variable is None if its computation returned None.
Value is none if its computation raised an exception.
- Move setup_filetable and clear_filetable methods from class dirtree_datafiles to its parent
 basic_datafiles; and improve the coding.
- Betther handling of lists of directories or files in the constructor of dirtree_datafiles.
- check for http: in a file specification, when cleaning up directory names.
- simplify switching between single- and multi- processor tests.
- This is set up for testing with a list of urls rather than a directory name.