Resolve "Add time resolved XAS metadata"

Closes #58 (closed)

Adds the ID24 laser as an NXsource.

We already had one NXsource group called source which functions as the primary source.

We could define sources like detectors, so with names source01, source02, ...

However in this case the ID24 laser source has an energy in J and a delay in ns as metadata. The fact that the units need to be fixed in ICAT makes this not a generic source anymore. So for now I used the fix group name laser but I'm not sure this is the best approach.