[FEATURE] Improve Divestment modeling and naming

[FBumann]

Prerequisites

• I have searched existing issues and discussions
• I have checked the documentation

Feature Category

API/Usability Improvement

Problem Statement

I find the divestment options in InvestParameters rather confusing.
I think it could be improved in several ways.

Proposed Solution

1. Add a previous_sizeparameter. Divestment only applies if this is greater 0 (similar to previous flow rate and previous state)
2. Rename divestto retired
3. Rename effects to be precise of their origin:

Current	Proposed	Rationale
fix_effects	effects_of_investment	More explicit about what these effects represent
specific_effects	effects_of_investment_per_size	Clearly indicates these scale with investment size
divest_effects	effects_of_retirement	Aligns with retired variable naming and is more descriptive

Use Case & Examples

investment = InvestParameters(
    effects_of_investment={...},      # one-time effects when investing
    effects_of_investment_per_size={...},  # effects that scale with size
    effects_of_retirement={...}       # effects when retiring the asset
)

### Desired API (Optional)

```python

Alternatives Considered

No response

Priority/Impact

None

Additional Context

No response

[FBumann]

Approved by @PStange