External application functions

Flextra and Flexpart

`flexpart_build_title()`	Generates title for plotting FLEXPART gridded output fields
`flexpart_convert_trajectory()`	Converts raw FLEXPART trajectory output to CSV
`flexpart_filter()`	Extracts fields from FLEXPART output GRIB files
`flexpart_prepare()`	Retrieves and prepares data for a FLEXPART simulation
`flexpart_release()`	Defines a FLEXPART release
`flexpart_run()`	Performs a FLEXPART simulation
`flexpart_total_column()`	Computes the sum/vertical integral of fields in a FLEXPART output GRIB file.
`flextra_group_get()`	Returns data for a FLEXTRA result group
`flextra_run()`	Performs a FLEXTRA simulation
`flextra_tr_get()`	Returns data for an individual FLEXTRA trajectory
`flextra_visualiser()`	Defines visualisation for FLEXTRA results

`met3d()`	Visualises GRIB data in Met3D
`met3d_prepare()`	Retrieves and prepare GRIB data for Met3D

Prepares and visualises GRIB data in Vapor

`scm_run()`	Runs the Single Column Model
`scm_visualiser()`	Defines visualisation for SCM output

`rttov_run()`	Runs the RTTOV model
`rttov_visualiser()`	Defines visualisation for RTTOV model output