ENH,RF: - Renamed "overrides" to "selectors".

        - Flatten dictionaries in main config file, e.g. "[struct_T1] k = v"
          is flattened to struct_T1_k = v
        - New gettuple method
        - Added ability to override values e.g. from command-line.
        - Basic type validation applied to overrides - error if their
          parsed type does not match the stored type