Spec v4 goals and features

[bjornharrtell]

• Dictionary for common strings (Idea: short strings & shared strings #43)
• Optimize for parallel I/O (Optimize for parallel I/O #82)
• Defined data boundaries to fx. allow additional data after feature data (probably needed for the above)
• Better extensibility with definitions of reserved use (fx. define how to handle unknown/new column types) (Provide general Column size meta #79)
• Consider memory model (Memory model future #106)
• Consider attribute data access (alignment, indexable)