[worker] JSON serialization version 4

[kif]

The main difference with version 3 is the separation of the description of the geometry in a poni structure instead of having it flat in the main.
Before:

{
'version': 3,
'poni_version': 2.1,
'detector': 'detector',
'detector_config': {'pixel1': 1e-4, 'pixel2': 1e-4},
[...]
}

After:

{
'version': 4,
poni: {
    'poni_version': 2.1,
    'detector': 'detector',
    'detector_config': {'pixel1': 1e-4, 'pixel2': 1e-4},
   [...]
},
[...]
}

Impacts every place where workers are used:

• worker
• io.integration_config --> doc goes there
• diffmap
• gui.diffmap_widget
• gui.IntegrationDialog
• test.test_worker
• app.integrate

[EdgarGF93]

#2249 work in progress

[kif]

TODO: check the progress