Add new readability and conversational metrics to evaluation module

- Added three new readability metrics:
  * Coleman-Liau Index: Grade level estimation using character counts
  * Linsear Write: Grade level based on easy vs difficult words
  * Dale-Chall Readability Formula: Difficulty assessment using familiar word list approximation

- Added five new conversational features:
  * Turn-taking ratio: Conversation balance using normalized Shannon entropy (0=monopolized, 1=balanced)
  * Question rate: Percentage of turns containing questions
  * Lexical diversity: Type-token ratio measuring vocabulary richness
  * Back-channel rate: Percentage of minimal response turns (acknowledgments)
  * Filler word density: Percentage of filler words like "like", "you know", "basically"

- Refactored metrics into semantic classes:
  * ConversationalFeatures: Dialogue structure and interaction dynamics
  * ReadabilityScore: Text complexity and reading difficulty
- All metrics support speaker filtering and flexible feature selection

Author: sergioburdisso