Just need the first element of constant parameter
The emulators are now passed as a dictionary of sza,vza keys (I'm assuming raa=0). This needs to change, because it's quite time consuming to do the whole emulation business. At present, we have 5degree quantised geometries, but might need to do something a bit better...
Did away with having the lidfb parameter: it's unneeded, and only required by prosail. This appears to work fairly well