Add new appraoch to specifying data in test single solver

[nicolasdlss]

Description Allows to use a pickle file from a previous calculation to set data in test_single_solver.

Users' experience affected? Additional feature added.

Other parts of the code affected? Not applicable.

Tests The new and old approach to set data in test_single_solver has been successfully tested.

Related issues Not applicable.

Checklist

• [x] Style guidelines
  1. CoCoNuT follows the PEP8 style guide.
  2. Comments should be lowercase.
  3. Errors should start with capitals, preferably without punctuation unless it's multiple sentences.
  4. Strings should use single quotes whenever possible.
• [x] Self-review of code performed
• [x] Code has been commented (particularly in hard-to-understand areas)
• [x] Documentation was updated correspondingly
• [x] New and existing unit tests pass locally with changes