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/notation.cpp

@@ -207,7 +207,7 @@ static string score_to_string(Value v) {
       s << "-#" << (VALUE_MATE + v) / 2;
 
   else
-      s << setprecision(2) << fixed << showpos << float(v) / PawnValueMg;
+      s << setprecision(2) << fixed << showpos << double(v) / PawnValueMg;
 
   return s.str();
 }