Change step increment for Mining productivity bonus from 2 to 10

[SquaredGrunt]

Fix for issue #141

[KirkMcDonald]

While this is correct for 0.17, it is not correct for 0.16, which remains the current stable version, as well as the calculator's default dataset.

It would be possible to change this property depending on the version, but it might be simplest just to wait until 0.17 is stable and I decide to remove 0.16 support from the calculator.

[SquaredGrunt]

I think 0.17 is stable and this could be merged now.