add support for sampling and rendering auxiliary data along trajectories

aim:

• extension for processing and rendering pre-computed auxiliary data along trajectories
• extension for sampling and rendering auxiliary data variables along trajectories (streamlines, pathlines) computed in Met3D

conceptual summary:

• add features for reading pre-computed auxiliary data along trajectories; aux-data and aux-data var names are stored in the trajectory object
• add features for sampling NWP data along trajectories computed in Met3D. The aux. data is stored in the trajectory object
• add features for rendering the color of particle positions/trajectory vertices and trajectory tubes according to the auxiliary data variables
• add features for writing aux data to file
• add GUI interface for: choosing how to render trajectories and which NWP variables to sample along trajectories

user interaction:

• pre-computed auxiliary data variables need to be included in a netcdf file as a variable with
  an attribute "auxiliary_data" set to "yes" and the same dimensions as the trajectory vertex information (ensemble, trajectory, time). Example (copy from ncdump): " float particleID(ensemble, trajectory, time) ; particleID:standard_name = "particle_ID" ; particleID:long_name = "particle id as aux data example" ; particleID:auxiliary_data = "yes" ; particleID:units = "unique number" ; "
• Sampling of NWP data along trajectories can be invoked by providing a comma-separated list of the NWP variables of interest in the GUI window also used for specifying other inputs for the computation of trajectories in Met3D
• Rendering of the color of particles and trajectory tubes according auxiliary data variables can be set in the GUI of the trajectory actor under rendering. One can choose to color according to pressure or auxiliary data. If one chooses to render according to auxiliary data, then a list of available auxiliary data variables appears for selection.

testing:

• the processing and rendering of pre-computed auxiliary data was tested in 2 ways: (i) write a small artificial trajectory file with simple auxiliary data (see above example ncdump); (2) post-process a recent NAME dispersal simulation output (outbreak of wheat rust in South-East Asia) containing several hundred thousand trajectories with various different auxiliary data variables
• the sampling and rendering of NWP data along trajectories computed in Met3D was tested for streamlines and pathlines using a very small sub-domain of a recent high-resolution ICON simulation, remapped to a regular lat-lon grid.
• all tests appeared fine in debug and release mode