[1.7] Migration Fix
Some fixes for immigration.
Currently in the original game, when a province has too many people to immigrate to, immigration will be completely impossible. This problem has been solved by modifying the immigration-related parameter settings. When the province has sufficient infrastructure, immigration can proceed normally.
【Specific principles】
1. From the feedback of players, there are two possible reasons for the current version not to migrate: ① Possible logical errors when exceeding the immigration limit; ② Paradox has set a minimum percentage for a single migration, and large groups of people cannot find suitable migration targets due to the limit of the immigration limit;
2. For ①, the immigration limit brought by infrastructure has been modified to make the total immigration limit less easy to reach;
3. For ②, MARKET_MIGRATION_MIN_POP_FRACTION_TO_MOVE has been modified, which limits the cancellation of all migrations when the total number of migrants is less than this percentage of the total population of the current province;
【Impact on the game】
1. Modifying MARKET_MIGRATION_MIN_POP_FRACTION_TO_MOVE may increase the burden on the CPU (so I just halved it instead of setting it to 0 roughly), because it will allow more pops to start migrating;
2. Modifying the immigration limit may make people feel that this work is similar to unbalanced MODs such as "crazy migration", but in fact it is not the case. The baseline value of Paradox immigration is one immigrant for every 100k population, which makes the immigration limit not so easy to reach. Paradox’s original intention of setting up an immigration limit may be that it does not want to allow the population of some turbulent provinces to lose too quickly, but due to disastrous numerical design, it caused more serious bugs (population migration in a large country like China is a scene worth noting in the Victorian era. People who are not familiar with history can learn about "going to Nanyang", "Chuang Guandong" and "Zou Xikou")
3. The current population migration level is within an acceptable balance. I used European countries to conquer some coastal areas of Qing. There are hundreds of thousands of unemployed people in the local province. After building railways, about a few thousand people will migrate to high SoL areas in my country every week. Some areas using the Green Grass decree may absorb a particularly large population because they can obtain population from multiple provinces.
