Debug_In_Release; improved export limiting; covert cell success rate reduced; HDI tweak; immigration and emigration modifications

- Debug_In_Release now compiles; updated the .gitignore to ignore files
in the Debug_In_Release folder.
- The effect of limiting net exports to production has been improved.
- Reduce the success rate bonus of having multiple covert cells in a
target country.
- The education component of the HDI now depends more on the previous
HDI; education component tends to plunge if the education budget factors
in too much.
- Fix a crash that may sometimes happen when the calculated emigration
level is negative (it should be capped to 0 minimum).
- Change the maximum immigration level to 2%, the same as the maximum
emigration level.