#885 - Delete validate_forecast()

[nikosbosse]

Description

This PR addresses some parts of #885.

As discussed there, validate_forecast() and assert_forecast() are mostly duplicates and we don't need it anymore now that we're revalidating the forecast object every time something changes.

I suggest exporting the missing functions when addressing #746 - that gives us more clarity on which functions need exporting (because the plan is to sort all functions necessary to create a new forecast type into separate files).

[Describe the changes that you made in this pull request.]

Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.33%. Comparing base (ff33cc7) to head (cc8cc9d).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #921      +/-   ##
==========================================
- Coverage   99.33%   99.33%   -0.01%     
==========================================
  Files          22       22              
  Lines        1650     1648       -2     
==========================================
- Hits         1639     1637       -2     
  Misses         11       11              

Flag	Coverage Δ
	99.33% <ø> (-0.01%)	⬇️

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