• Updated Size.from_string() to accept 0 size without measuring unit
• Replaced ValueError with CaptionReadSyntaxError for invalid sizes
  passed to Size.from_string()
• Updated DFXPReader timestamp validation according to TTML time
  expression specs
• Updated flashing cues validation for SCCReader to raise a
  CaptionReadTimingError
• Fixed SCC translator not recognising special and extended characters
• Raise CaptionReadTimingError for missing 'start' on SAMIReader