Concatenation

This MR adds workflow sort-concat to and embeds this workflow in the pipeline. It comprises three parts. The first part is a step sort-concatmap, which sorts a list of measurement sets into frequency groups by timestamp. These groups are then, in the second part, each concatenated into bands via DP3, using a regular frequency grid. This is done in a concatenate subworkflow.

The concatenate workflow, in turn, has three steps. The first step filter_ms_group determines which measurement sets correspond to the previously selected subbands. In the second step, DP3 performs the actual concatenation for each frequency band.

In order to facilitate logging the log files for each of bands are concatenated in the third step of sort-concat. The final part collects all log files into a separate directory. Finally, this directory, together with the newly-created bands, are passed along as output.