Add option to save catalog object

[AdrienWehrle]

Some users might want to have access to specific platforms (e.g. to distinguish between data from Sentinel-2 A and B) or other tile-specific details. Let's have an option to output the full catalog object.

Metadata are saved e.g. here:

earthspy/earthspy/earthspy.py

Line 253 in 9ee52fb

self.metadata = [list(iterator) for iterator in search_iterator]

and for instance, the platform(s) could be obtained with

[m["properties"]["platform"] for m in self.metadata[0]]

Such metadata could actually be saved straight away as metadata in the geotiff files, as shown here.

see #84 to modify file save.