Skill level names

[Philippe Michel]

Currently gnubg uses relatively obscure names for its skill levels. 

In the player settings, which is stronger between World Class, Supremo, 
Grandmaster is not obvious at first sight.

In the analysis results, Supernatural and Awful are a bit awkward (the 
latter is excluded from the Japanese translation!)

Would you mind if we simply used "<n>-ply" as regular levels (keeping 
only one at 2-ply) for bot player settings and something like "0-ply 
beginner", etc... for the weakend levels.

For the analysis results we could use the denominations from BMAB 
(https://bgmastersab.com/about) without the 0/1/2/3 subdivisions:
error rate <= 5 Super Grandmaster
           <= 8 Grandmaster
           <= 13 Master
           <= 20 Advanced
           <= 32 Intermediate
           else either Casual or Beginner