Fix bugs in storage operation

[adamsjohanna]

• Use abs() for marginal_costs in flexable_storage.py
• Use the right times in calculate_generation_cost() in base.py
• Delete empty orders in formulate_bids in units_operator.py
• Bidding positively on CRM market

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (f502c48) 83.00% compared to head (ea9b118) 83.12%.

Files	Patch %	Lines
assume/strategies/flexable_storage.py	50.00%	1 Missing :warning:
assume/units/storage.py	97.05%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #291 +/- ## ========================================== + Coverage 83.00% 83.12% +0.12% ========================================== Files 40 40 Lines 4530 4564 +34 ========================================== + Hits 3760 3794 +34 Misses 770 770 ``` | [Flag](https://app.codecov.io/gh/assume-framework/assume/pull/291/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/291/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=assume-framework) | `83.12% <95.00%> (+0.12%)` | :arrow_up: | 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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