Feature: Configurable game ending year

Functionally reverts 683b65ee1

Fix: Crash when attempting to load old save game with GRFs set

GroupStatistics pool was not initialised before trying to delete vehicles (specifically, trams with no tram track)

Fix #7400: Water class for tree tiles was not converted for old saves preventing industry creation.

As the information is always available from the tree ground type, unconditionally
update the map array for tree tiles.

Feature: Industries with neutral stations (e.g. Oil Rig) only supply/accept cargo to/from their neutral station. (#7234)

This change is a controlled by a game setting, located under Environment ->
Industries which allows toggling the behaviour. It defaults to enabled.

"Company stations can serve industries with attached neutral stations"

When enabled, industries with attached neutral station (such as Oil Rigs) may
also be served by company-owned stations built nearby. This is the traditional
behaviour.

When disabled, these industries may only be served by their neutral station.
Any nearby company-owned stations won't be able to serve them, nor will the
neutral station serve anything else other than the industry.

Add: Option for population-linear town cargo generation

Introduce a new default algorithm for town cargo generation (passengers and mail), and a game setting to choose between the new and original algorithm.

The original town cargo generation algorithm has the property of the generated amount relating to the square of each building's population, meaning large towns easily produce more cargo than can realistically be transported. The problem is excessive cargo is amplified if playing with cargodist.

The new algorithm introduced instead has a linear relation to the population. The result is that smaller towns will produce slightly more cargo, while the largest towns will produce about a fourth of what they would with the original algorithm.

Existing savegames will use the original algorithm, while new games will default to the new algorithm.

Codechange: Convert saveload numbers to enum values.

(This was mostly achieved with a few in-place regexes)

(svn r25508) -Change: split unit localisation choice into a choice per type of unit, and move it to the advanced settings
-Feature [FS#5482]: have tractive effort in imperial (lbf) and metric (kgf) units
-Feature: have weights and volumes in imperial units (short tons, gallons)