Write out model fields

• Change output naming to include platform / instrument
• Allow output files to be written for multiple platforms
• 'NOPAMTRA' instrument writes out model fields so that PAMTRA can be run externally.
• EXPERIMENTAL: can specify 'curtains' as instruments - defined by a trajectory file or by pairs of points between which a great circle is drawn. The curtains can have unique output frequencies set by write_every_s in the platform dictionary in pamtra.yml. I am classing this as experimental for now as it seems that not all point along the curtain are being written - I need to figure out why this is.