separate predict and simulate functions

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 94.49%. Comparing base (51798c0) to head (593e423).

Files	Patch %	Lines
src/forecast.jl	87.50%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #19 +/- ## ========================================== - Coverage 94.74% 94.49% -0.25% ========================================== Files 13 13 Lines 1503 1508 +5 ========================================== + Hits 1424 1425 +1 - Misses 79 83 +4 ```

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