fix focus always be a penalty when install GUNMOD with INSTALL_DIFFICULT

[osuphobia]

Summary

Bugfixes "fix focus always be a penalty when install GUNMOD with INSTALL_DIFFICULT"

Purpose of change

In gunmod_installation_odds, it is said that focus is either a penalty or bonus of at most +/-10%, but that's not how it works right now. Actually, with std::min( std::max( get_focus(), 140 ), 60 ), focus always gives you a -10% penalty.

Describe the solution

Fix it by using std::clamp( get_focus(), 60, 140 ).

Describe alternatives you've considered

Not only fix this minor issue, but also refactor gunmod_installation_odds.

Testing

TBD

Additional context