New tech buying code for mil tech doesn't take destocking into account

[jobbish-sql]

caused by #24, would ideally be fixed before it merges

Right now the code to determine how profitable mil tech is only looks at current expenses, so countries aren't likely to buy a lot of it. Perhaps the following scheme is good enough?

• the EE api takes in a pm_destock_amount parameter that's only used for mil tech and when turn bucket = 100 (since the mil tech only pays itself off at the very end)
• the ee_npc code sets the parameter as the amount of cash that's above 1 billion? if a country is hanging around with a billion cash at the start of turns they'll probably be able to destock at least a billion of it, by my reckoning. this also prevents countries from prioritizing mil tech too early in the set

[jobbish-sql]

I think I have a reasonable fix for this in #24 now, so closing