Improvements for SPARTA data processing

[avigan]

• Support ECMWF forecast model (https://www.ecmwf.int/en/forecasts). Already available in @jmilou/sparta code, but relies on a manual download and pre-processing of the data
• Multi-OB SPARTA analysis. Again, already available in @jmilou/sparta code but not currently supported here
• Compute Strehl on DTTS images