Reintroduce tropism to kingdanger

Tropism in kingdanger was simplified away in this pull request #1821.
This patch reintroduces tropism in kingdanger with using quadratic scaling.

Passed STC http://tests.stockfishchess.org/tests/view/5bf7c1b10ebc5902bced1f8f
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 52803 W: 11835 L: 11442 D: 29526

Passed LTC http://tests.stockfishchess.org/tests/view/5bf816e90ebc5902bced24f1
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 17204 W: 2988 L: 2795 D: 11421

How do we continue from there?

I've recently tried to introduce tropism difference term in kingdanger which
passed STC 6 times but failed LTC all the time. Maybe using quadratic scaling
for it will also be helpful.

Bench 4041387

src/evaluate.cpp

@@ -475,6 +475,7 @@ namespace {
                      +  69 * kingAttacksCount[Them]
                      + 185 * popcount(kingRing[Us] & weak)
                      + 150 * popcount(pos.blockers_for_king(Us) | unsafeChecks)
+                     +       tropism * tropism / 4
                      - 873 * !pos.count<QUEEN>(Them)
                      -   6 * mg_value(score) / 8
                      +       mg_value(mobility[Them] - mobility[Us])