Implement parallel input of data files in example

[pierrepebay]

Have 4 ranks reading their own part of data input and have 1 rank generating the image.

[pierrepebay]

When trying to get a rank that doesn't exist in an info object, the exception could be outputted in a more explicit and understandable way.

• Current error:
  
• Possible better handling:
  Exception in unordered_map::at : key not found : rank 2 does not exist in this info

[ppebay]

This issue appears to combine

• an enhancement ("Have 4 ranks reading their own part of data input and have 1 rank generating the image.")
• a bug (the exception could be outputted in a more explicit and understandable way)

These two should be disentangled and possibly separated in two different issues (one which may no longer be relevant).