Enables running ERT in ensemble_experiment mode

[edubarrosTNO]

Enables flownet ahm to run ERT in ensemble_experiment mode to facilitate evaluation of prior ensemble only

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Contributor checklist

• [x] :tada: This PR closes #447.
• [x] :scroll: I have broken down my PR into the following tasks:
  • [x] Change config_parser to allow for ert.ensemble_weights to become optional entry in the configuration file
  • [x] Change run_ahm to run ERT in ensemble_experiment mode if no weights are provided
• [ ] :robot: I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR.
• [x] :book: I have considered adding a new entry in CHANGELOG.md.
• [x] :books: I have considered updating the documentation.