Improve STAC generation

[pont-us]

STAC generation is still rather crude: constant placeholder values are used for bounding box and timespan. At least these problems should be fixed, and output should be reviewed to see if there are any other obvious corrections or improvements to make.

[Hneuschmidt]

As a simple improvement I suggest allowing users to add specific metadata through attributes on the dataset.
If more output product types are added as suggested in #83 it should be considered how to provide a consistent interface for different output types.

[Hneuschmidt]

As a minimum useful addition, users should be able to set the geometry attribute of the output STAC item.

In the future, we could investigate how to discover the geometry automatically for a number of common cases.