Rework civ ability, add compatibility with Dvaravati

Ability: Rivers of Fish, Fields of Rice
Old: +1 Food on Farms, Plantations, Camps, and Fishing Boats. +1 Production on all Improvements.
New: Settlements with their City Center adjacent to a Navigable River receive +2 Food and +1 Production on Farms, Plantations, Camps, and Fishing Boats. Other Settlements receive +1 Food on these Improvements.

Unique Unit: Pho Kha
Old: Gain 25 Gold and 10 Influence for every Resource acquired when creating a naval Trade Route.
New: Gain 25 Gold and 5 Influence for every Resource acquired when creating a Trade Route.

Na Khun Tradition
Old: Settlements with their City Center adjacent to a Navigable River receive +0.5 Food and +0.5 Production for every Rural Population.
New: Settlements with their City Center adjacent to a Navigable River receive +20% Food and +10% Production.

Khlang Kosathibodi Tradition
Old: +10 Gold and +4 Influence for every Trade Route.
New: +10 Gold and +2 Influence for every Trade Route.

Add compatibility with Dvaravati mod


Fix bugs that cause validation failure

- Pho Kha is now available at the start of the age.