Use FIDE table for elo calculation

[aarew12]

Close #579

[ornicar]

I think both List[Range.Inclusive, BigDecimal] and Map[Range.Inclusive, BigDecimal] are suboptimal, because we're traversing it anyway, and because these ranges are superfluous.

Instead of

val conversionTableFIDE: Map[Range.Inclusive, BigDecimal] = Map(
  (0 to 3)              -> 0.50,
  (4 to 10)             -> 0.51,
  (11 to 17)            -> 0.52,

[...]

val expectedScore = conversionTableFIDE.collectFirst {
  case (range, pd) if range.contains(absRatingDiff) => pd

I would prefer to see

val conversionTableFIDE: List[Int, BigDecimal] = List(
  3              -> 0.50,
  10             -> 0.51,
  17             -> 0.52,

[...]

val expectedScore = conversionTableFIDE.collectFirst {
  case (upperBound, pd) if upperBound >= absRatingDiff => pd

[lenguyenthanh]

I would prefer to see

val conversionTableFIDE: List[Int, BigDecimal] = List(
  3              -> 0.50,
  10             -> 0.51,
  17             -> 0.52,

[...]

val expectedScore = conversionTableFIDE.collectFirst {
  case (upperBound, pd) if upperBound >= absRatingDiff => pd

I was thinking of a Map with multiple keys point to the same value:

val conversionTableFIDE: Map[Int, BigDecimal] = mapOf(
0 -> 0.5,
1 -> 0.5,
2 -> 0.5,
3 -> 0.5,
...
15 -> 0.52,
16 -> 0.52,
17 -> 0.52,
....
391 -> 0.91
)

and we can do (pseudo code)

  def getExpectedScore(ratingDiff: Int): BigDecimal =
     conversionTableFIDE.getOrElse(ratingDiff, 0.92)

[lenguyenthanh]

gentle ping @ornicar for reviewing