Merge dataset arrays

(in a SQL equijoin)

Function JOINDST implements SQL 'join' operation with a single equality condition.
http://en.wikipedia.org/wiki/Join_(SQL)

It has been tested with numeric keys only, and not optimized; improvements are enthusiastically welcomed.

See also the matrix version JOINMAT.

PS. Note that output rows are sorted, and Inf values are treated in the same way as (but are distinguished from) NaN values.