Fix missing fixed costs and fix forecaster

[nick-harder]

-fix missing fixed cost in marginal cost calculations -> now power plant and storage units consider both variable and fixed cost

• before all calculations were missing fixed costs completely since all input files provide only fixed costs -fix forecaster keeping old values even if new values are given -> now values are overwritten -fix learning_strategy missing cost calculations -fix tests to account for missing fixed cost in marginal cost

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (7380ef5) 83.86% compared to head (5098d0d) 83.86%.

Files	Patch %	Lines
assume/strategies/flexable_storage.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #295 +/- ## ======================================= Coverage 83.86% 83.86% ======================================= Files 40 40 Lines 4549 4549 ======================================= Hits 3815 3815 Misses 734 734 ``` | [Flag](https://app.codecov.io/gh/assume-framework/assume/pull/295/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=assume-framework) | Coverage Δ | | |---|---|---| | [pytest](https://app.codecov.io/gh/assume-framework/assume/pull/295/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=assume-framework) | `83.86% <96.42%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=assume-framework#carryforward-flags-in-the-pull-request-comment) to find out more.

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