Ais update

[cherrylchico]

For the port arrivals and trade volume

• cited our two unpublished works (Samoa paper and trade estimation for pacific), with both of us as authors
• included short narrative on methods for port arrivals with proper citation
• added short theoretical foundation for trade volume
• fixed the references, included only cited in document

For hhi

• wrote short narrative on HHI definition
• the suggested graph is the yearly HHI per country per major vessel types (cargo, passenger, tanker, and fishing)