Add PrTracks to v1::Track converters

Write converters that directly convert PrTracks to v1::Track, without going over v2::Track. Needed for: LHCb#136

• PrDownstreamTracks -> v1::Track
• PrSeedingTracks -> v1::Track
• PrLongTracks -> v1::Track
• PrVeloTracks -> v1::Track
• PrUpstreamTracks -> v1::Track
• PrFittedForwardTracks -> v1::Track