join: Emit an error if the input is not sorted [feature request]

[sjackman]

mlr silently gives the wrong answer if the input is not sorted. An error message would be quite helpful. Alternatively, restart the join as though mlr join -u were specified, if it's possible to rewind the inputs (neither are pipes).

[johnkerl]

This is by design, to mimic the system join command.

Probably best thing to do is make -u the default.

[johnkerl]

... which I personally would not mind at all. And it wouldn't be backwards incompatible.

[sjackman]

I'm fine with making -u the default. An option the invokes the previous behaviour might be helpful to some.

[johnkerl]

OK this done in head; will go out with next release. Thank you!! :)

[sjackman]

Thanks again, John!