Backgammon notation v2

[jamesdfrost]

Changes to use Backgammon standard narrative formats for human readable notation, as follows:

• Always show the numbering going from Bar->24->0->Off, irrespective of which player is moving.
• Show the start position followed by end position, instead of start position, number of moves. E.g. 24/22 10/8
• Show hits with an asterisk, e.g. 9/7*.
• Order the moves by highest number first, e.g. 22/7 10/8 not 10/8 22/7. Not an official requirement, but seems to be standard convention.
• Show duplicate moves as 10/8(2) instead of 10/8 10/8.
• Show moves on a single piece as 10/8/5 not 10/8 8/5
• Also introduced tests to check for the above scenarios.

Files changed - backgammon.cc, backgammon_test.cc and backgammon.txt (for the playthrough tests)

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[lanctot]

@jamesdfrost , thanks. Looks good, I'll important it and hoepfully get it into our next update.

Seems like you need to sign the CLA (I think you must have had to do this for the first PR too?)

[jamesdfrost]

I've signed it with my github account jamesdfrost, but VScode seems to have uploaded these files using "James Frost" - I'll fix and resubmit.