Merge matrices

(in a SQL equijoin)

Function JOINMAT 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 dataset-array version JOINDST.

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