sigma symbol in python commands

Some of the python commands involve typing in a sigma symbol, it could be worth adding a step to rename the layers (e.g. 'σ' -> 'sigma'), so that it is easier to type in commands like the one below in the python console.

```
# Get the image data for the blurred nuclei
blurred = viewer.layers["nuclei_gaussian_σ=3.0"].data
```