Provide conversion of sits objects to/from other R packages

[gilbertocamara]

Describe the requested improvement
Users would benefit from the capacity to convert of sits objects to/from other R spatial packages, such as terra, stars and sf. Converting from a stars object to a sits cube could facilitate user experience, particularly for local files.

Associated sits API function
sits_cube()
sits_as_sf()

Additional context
Progress on this work should be reported to @mdsumner @edzer @loreabad6 @Nowosad @mpadge @Nowosad @alexgleith

[gilbertocamara]

For information to @mdsumner @edzer @loreabad6 @Nowosad @mpadge @Nowosad @alexgleith:

sits_as_sf() is already available in version 1.5.3-dev (development version)

[gilbertocamara]

For information to @mdsumner @edzer @loreabad6 @Nowosad @mpadge @Nowosad @alexgleith:

sits_as_stars() is already available in version 1.5.3-dev (development version)

[gilbertocamara]

For information to @mdsumner @edzer @loreabad6 @Nowosad @mpadge @Nowosad @alexgleith:

sits now provides functions sits_as_sf, sits_as_stars and sits_as_terra to convert from sits objects to stars, terra, and sf one. As suggested by @edzer, we will also include a st_as_stars.raster_cube function so that stars users can directly import data cubes from sits

The new functions can be assessed and tested in the "dev" version:

devtools::install_github("e-sensing/sits@dev")