Use double everywhere

Rationale:

- Speed of double and float is about the same (not on the hot path anyway)

- Double makes code prettier (no need to write 1.0f, just 1.0)

- Only practical advantage of float is to use less memory, but since we never
  store large arrays of double, we don't care.

No functional change.

src/timeman.h

@@ -26,7 +26,7 @@
 class TimeManager {
 public:
   void init(const Search::LimitsType& limits, int currentPly, Color us);
-  void pv_instability(float bestMoveChanges);
+  void pv_instability(double bestMoveChanges);
   int available_time() const { return optimumSearchTime + unstablePVExtraTime; }
   int maximum_time() const { return maximumSearchTime; }