netcdf support

[tjhei]

As discussed in the call today:

• enable netcdf in the tester (adjust docker images)
• add it to the installation instructions / candi.cfg / check that the ubuntu package works
• make it a required dependency (yes?)
• allow it to be used in all ascii data plugins
• add a test prm that uses it
• consider renaming ascii data to structured data in the parameter names for 3.0
• decide if everybody, once per node, or rank 0 needs to load the file