Yml instrument section

This pull request implements a change that we intended to make to the config file for the 0.2 branch, but was overlooked until now. Config files that used to read:

```yaml
detector:
  parameters_old: CeO2.par # the old detector spec, produced by the GUI
  parameters: CeO2.yml
  pixels: # this section required to convert from old detector spec
    rows: 2048
    columns: 2048
    size: [0.200, 0.200] # [row height, col width] mm
```

will need to be updated to read:

```yaml
instrument:
  parameters: CeO2.yml
  detector: # this section required to convert from old detector spec
    parameters_old: CeO2.par # the old detector spec, produced by the GUI
    pixels: 
      rows: 2048
      columns: 2048
      size: [0.200, 0.200] # [row height, col width] mm
```

Once pulled into 0.2.x, I'll tag 0.2.2, and also merge into praxes/master.

@mwiebe @joelvbernier