Refactor discretization.

[thorstenhater]

This PR rectifies a long-standing issue since the introduction of decor:
CV discretization was treated as a defaultable value instead of a cell property, along with bio-physical properties.

This lead to some awkward facts:

• Intermingling of bio-physics and numerical settings
• Discretization is treated like a default without override.
• Discretization cannot be changed once set, e.g. if read from .acc, requiring the copy+update of a decor instead

Thus:

• Discretization can now be set during cable_cell construction
• It can be read and updated later
• Refactor cv_policy to use type erasure like other, similar constructions, e.g. schedules.
• Hide internal cv_policy constructors.

⚠️ Breaks .acc by upgrading from 0.9 to 0.10 ⚠️

Closes #1408