[RL] Add periodic boundary conditions to all grid-based competition modules

[Lgz-tud]

Summary

Part of #401

• Lift PBC to base class: ResourceModel.makeGrid() now stores domain size (_lx, _ly) and parses the optional <periodic_boundary> XML tag. New wrapDistance(dx, dy) method applies the minimum image convention.
• Refactor FON: Remove FON's own PBC parsing, use inherited base class infrastructure instead.
• Add PBC to AsymmetricZOI, SymmetricZOI, SaltFeedbackBucket: Each module accepts <periodic_boundary>True</periodic_boundary> in its XML config. Distance calculations are wrapped via wrapDistance().

Usage

Add to any grid-based resource module's XML config:

<periodic_boundary> True </periodic_boundary>

[mcwimm]

Very cool! Do you think it's possible to define the periodic boundary only once?

<resources>
    <periodic_boundary> True</periodic_boundary>
    <aboveground>
        <type> Default </type>
    </aboveground>
    <belowground>
        <type> Default </type>
    </belowground>
</resources>

[Lgz-tud]

Thanks for the suggestion — agreed, defining it once at the level is much cleaner.

• <periodic_boundary> is now read once at the level and propagated to both above- and below-ground module configs.
• Defining the tag inside or raises a ValueError with a clear message pointing to the new location.
• FON_PBC_CI benchmark XML migrated to the new layout (reference output unchanged).
• Module docs (ResourceModel, AsymmetricZOI, SymmetricZOI, SaltFeedbackBucket) updated with the new example.

[mcwimm]

Very cool 👍

[mcwimm]

Shouldn't "periodic_boundary" be removed from the tag dictionaries of all resource modules?

[Lgz-tud]

Two changes since @mcwimm last review:

1. Rebased onto master (which now includes [RL] FON: add optional C++ backend #400's C++ backend).

2. Removed periodic_boundary from per-module tag dictionaries . Base class ResourceModel.getInputParameters auto-injects it now.

All benchmarks pass. Ready for re-review.