Add a per-mob configuration option to change how power costs are calculated when mass upgrades are used.

By default a linear function will be used, where each mob costs the same as the last.
Extra option will be available to set that function to do other things, such as double the cost for each extra mob.

Initially there will be
linear - each mob costs the same
base2 - mob cost = mob cost * (2^(mob - 1))
base 3 - mob cost = mob cost * (3^(mob - 1))

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