Add gather_statistics command that allows gathering statistics from a .bin or .binpack file. Initially only support position count.

docs/stats.md

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+# Stats
+
+`gather_statistics` command allows gathering various statistics from a .bin or a .binpack file. The syntax is `gather_statistics (GROUP)* input_file FILENAME`. There can be many groups specified. Any statistic gatherer that belongs to at least one of the specified groups will be used.
+
+Simplest usage: `stockfish.exe gather_statistics all input_file a.binpack`
+
+## Groups
+
+`all`
+
+ - A special group designating all statistics gatherers available.
+
+`position_count`
+
+ - `struct PositionCounter` - the total number of positions in the file.