UCI_Elo implementation (#2225)

UCI_Elo implementation (#2225)

This exploits the recent fractional Skill Level, and is a result from some discussion in #2221 and the older #758.

Basically, if UCI_LimitStrength is set, it will internally convert UCI_Elo to a matching fractional Skill Level.
The Elo estimate is based on games at  TC 60+0.6, Hash 64Mb, 8moves_v3.pgn, rated with Ordo, anchored to goldfish1.13 (CCRL 40/4 ~2000).
Note that this is mostly about internal consistency, the anchoring to CCRL is a bit weak, e.g. within this tournament,
goldfish and sungorus only have a 200Elo difference, their rating difference on CCRL is 300Elo.

I propose that we continue to expose 'Skill Level' as an UCI option, for backwards compatibility.

The result of a tournament under those conditions are given by the following table, where the player name reflects the UCI_Elo.

   # PLAYER          :  RATING  ERROR  POINTS  PLAYED   (%)  CFS(%)
   1 Elo2837         :  2792.2   50.8   536.5     711    75     100
   2 Elo2745         :  2739.0   49.0   487.5     711    69     100
   3 Elo2654         :  2666.4   49.2   418.0     711    59     100
   4 Elo2562         :  2604.5   38.5   894.5    1383    65     100
   5 Elo2471         :  2515.2   38.1   651.5     924    71     100
   6 Elo2380         :  2365.9   35.4   478.5     924    52     100
   7 Elo2289         :  2290.0   28.0   864.0    1596    54     100
   8 sungorus1.4     :  2204.9   27.8   680.5    1596    43      60
   9 Elo2197         :  2201.1   30.1   523.5     924    57     100
  10 Elo2106         :  2103.8   24.5   730.5    1428    51     100
  11 Elo2014         :  2030.5   30.3   377.5     756    50      98
  12 goldfish1.13    :  2000.0   ----   511.0    1428    36     100
  13 Elo1923         :  1928.5   30.9   641.5    1260    51     100
  14 Elo1831         :  1829.0   42.1   370.5     756    49     100
  15 Elo1740         :  1738.3   42.9   277.5     756    37     100
  16 Elo1649         :  1625.0   42.1   525.5    1260    42     100
  17 Elo1558         :  1521.5   49.9   298.0     756    39     100
  18 Elo1467         :  1471.3   51.3   246.5     756    33     100
  19 Elo1375         :  1407.1   51.9   183.0     756    24     ---

It can be observed that all set Elos correspond within the error bars with the observed Ordo rating.

No functional change