Refactor pop_size_t to be type-safe

[Spartan322]

Move pop_size_t to population/PopSize.hpp
Add equalable to Concepts.hpp
Add expect_strong_typedef to NodeTools.hpp
Add && equalable<ValueType, OtherValueType> requirement to IndexedFlatMap's divide_assign_handle_zero

[schombert]

You are probably flying close to the sun by picking uint32_t to store the pop size in. That comes out to a maximum value of about 4 billion, which is obviously more than enough for any single pop. However, you are likely to want to add up pop sizes for various demographic numbers, including over an entire nation in some cases, which has to accommodate world conquest. And even then 4 billion is probably enough for the base game, especially since they are real population numbers divided by 4. However, there are mods set in other periods (for example the modern day mods that cover the near future), and it is easy to imagine that the world population in those mods may at times exceed 16 billion given how pop growth works if you play them long enough.

[Spartan322]

This is probably a good argument to separate individual pop size storage and pop displayable size. Even then the display size would potentially have two versions even then, a signed and unsigned version as a 64-bit value, since there are some cases where negative values are necessary and others where it isn't.