Adds resampling of observations

[edubarrosTNO]

Adds resampling of observation dates at requested frequency by finding the nearest date among existing observation dates.

---

Contributor checklist

• 🎉 This PR partially addresses Too many observations used when history matching Norne #206.
• 📜 I have broken down my PR into the following tasks:
  • Resampling observation dates based on requested frequency.
  • Adds (optional) resampling entry to the config parser.
• 🤖 I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR.
• 📖 I have considered adding a new entry in CHANGELOG.md.
• 📚 I have considered updating the documentation.

[wouterjdb]

LGTM. Two minor comments:

1. See review, I suggest to indicate that this new function is an internal function by using an underscore. This also makes it clearer for the reader it is not really a test as such.
2. You now always test with resampling. Could you maybe run, in that same test, twice the same function. Once with resampling (and check that the output is as expect) and once without resampling (and check that the output is as expected).

[wouterjdb]

LGTM 🚀