ENH: allow FeatureTable[RelativeFrequency] for filter-features

[VinzentRisch]

PR #358 adds core scores, and downstream workflows need to pass relative-frequency-style values as metadata input so features can be filtered by core score.

filter-features currently only accepts FeatureTable[Frequency] and uses min_frequency / max_frequency integer thresholds. This should also support FeatureTable[RelativeFrequency], where the corresponding thresholds are fractions from 0 to 1.

Proposed change:

• Allow filter-features to accept both FeatureTable[Frequency] and FeatureTable[RelativeFrequency].
• Rename min_frequency / max_frequency to min_abundance / max_abundance.
• Use a TypeMap so frequency tables allow whole-number thresholds from 0 to infinity, while relative-frequency tables allow fractional thresholds from 0 to 1.
• Update the private filtering helper names to reflect abundance/fraction semantics.
• Add a regression test covering fractional filtering on a relative-frequency table.