Run macro on configurable-size disjoint subsets of marked species

[westram]

Description:

Currently the feature Execute with each marked species runs a macro once per marked species (subset size = 1). Add support for invoking the macro on disjoint subsets of marked species with a configurable subset size.

Wanted behavior:

• Configurable maximum subset size N (default 1).
• Macro invoked once per disjoint subset of up to N marked species.
• A subset will become fully marked before the macro is called for that subset.
• Because ARB cannot select multiple species simultaneously, support to
  • either select exactly one representative per subset (e.g., first in order), or
  • deselect species.

Required GUI/config:

• subset size (default: 1).
• selection mode (default: representative)

(defaults ensure backward compatibility)

Constraints:

Form subsets deterministically for reproducible results:

• use order
• reduce subset size N such, that
  • the number of resulting subsets does not get increased,
  • but the size difference between the last and all other subsets is minimized.