Covert action/cell improvements; internal laws window changes; HDI changes

- When executing a covert action, if the mission was a success but the
player was found out, then the mission outcome window will explicitly
say that.
- The covert cells listing window now shows the number of Elite cells
required to achieve at least 94.9% security.
- Immigration and emigration levels are now shown to a precision of 3
decimal places.
- The internal "population level" stat can now take on non-integer
values.
- More covert cells in a target country now contribute to the success
rate of any missions.
- Redid and rewrote the code to calculate HDI.