Gaussian blur to the powder image

 Added a new function estimate_dist_center which will give the sample detector distance and calibrated center using other functions.  Added a gaussian blur to the powder image.
I suggest something like

```python
if sigma is None:
    sigma = _SIGMA_VAL.get(st_name, None)
if sigma:
    p_image = ndimage.gaussian_filter(input_image, sigma)
else:
    p_image = np.array(input_image)  # or just p_image = input_image if we are not worried about mutating the input
```

so that you can put 'falsy' in the `_SIGMA_VAL` dictionary if you want to be able to skip the filtering step and pass in any False value (but `None`) to disable the filtering.  Using `get` instead of `[]` lets us specify a default value to return in the case that the key does not exist so this does not fall on it's face if the standard does not exist in _this_ dictionary.

This might be getting too clever, but I don't think so