fix: deal with ImageCollection as with Folder

[12rambau]

Fix #27

The previous implementation was enable to deal with imageCollection objects. As such if someone wanted to test an ImageCollection asset he was force to run the ImageCollection creation from the code instead of the tree dict description.

In this PR I introduced a new type keyword that need to be specified when crzating containers "IMAGE_COLLECTION" or "FOLDER". like in the documentation example:

{
  "folder::Folder": {
    "image": ee.Image(1).clipToBoundsAndScale(point.buffer(100), scale=30),
    "fc": ee.FeatureCollection(point),
  },
  "image_collection::ImageCollection": {
    "image1": ee.Image(1).clipToBoundsAndScale(point.buffer(100), scale=30),
    "image2": ee.Image(1).clipToBoundsAndScale(point.buffer(100), scale=30),
  }
}

Only "Folder" and ImageCollection" types are supported (and to my knowledge they are currently the only 2 types of containers)

To avoid non-regression an non specified container type will be treated as a Folder.