[ENH] New predict functions, and docs on custom trend

[felipeangelimvieira]

Added

• New predict functions to handle MCMC samples
• Docs on how to build custom samples
• Scale param, so that users can force a scale value instead of relying on internal procedure (e.g. making scale=1 would simply bypass scaling). This can be useful in the case of custom trend and when the user prefers to handle it manually.

Fixed

• poetry.lock deleted, since it freezes libraries during CI processes.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.95652% with 9 lines in your changes missing coverage. Please review.

Project coverage is 92.35%. Comparing base (50ba6de) to head (e066cca).

Files	Patch %	Lines
src/prophetverse/sktime/base.py	87.27%	7 Missing :warning:
src/prophetverse/sktime/multivariate.py	66.66%	1 Missing :warning:
src/prophetverse/sktime/univariate.py	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #58 +/- ## ========================================== + Coverage 90.93% 92.35% +1.42% ========================================== Files 19 19 Lines 993 1047 +54 ========================================== + Hits 903 967 +64 + Misses 90 80 -10 ```

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