GSAS Reader/ Writer

mild preferance to do this block of logic with a dictionary look up

```python
func_look_up = {'std': _get_std_data, 'esd': _get_esd_data, 'fxye': _get_fxye_data}
try:
    tth, intensity, err = func_look_up[mode](file)
except KeyError:
   raise ValueError('...')
```