Add library of relevant SRFs

[robintw]

These should be able to be taken from the SRF definitions in the Py6S code, probably with some automatic processing of the Py6S source

[robintw]

• [x] Landsat OLI (from Py6S)
• [x] RapidEye (from Py6S)
• [ ] Pleiades (from Py6S)
• [ ] Sentinel-2 (where???)
• [ ] Landsat TM + ETM (from 6S source)