Merge scans

This MR reworks the starting of xsocs to add a multi application launcher.

It adds a second application "xsocs concat" to merge multiple master h5 files into one. As is, it is very basic: It creates a master file with external links to all scans in input file. It is a first step for merging scans (#8), as just doing this might cause issues afterwards (e.g., order in IntensityView, to be checked).