Provide a reader for a tileset

[EricBoix]

User story/Use caser

Having a reader at hand would allow:

• to convert the projection system of tileset,
• to display the information of a given tileset,
• to compare two tilesets...

Up to now, only writing a tileset is possible. Provide such a tileset reader

Notes

• The code for reading a Tile Content (i.e. either a b3dm or a point set) already exists: it is the TileContentReader class
• What is missing is the
  • parsing of the tileset description as written in json
  • its breakdown into sub-objects e.g. Tile that itself breaks down into Tile content, Bounding Volume
  • the conversion of this breakdown into Python native objects (in memory)
• Note that the current definition of the Tile object doesn't point to its Tile Content as a Python object but directly as either a b3dm or a pnt object. Define a TileContent class and have Tile use instances of that class.
• It would be the role of this new TileContent class to incorporate the TileContentReader class class (probably as a reader method).

Concerning the breakdown parsing system

In order to validate the json produced out of an in memory tileset (prior to writing a tileset) we a use a break down system provided by the jsonschema library: refer to the code of schema_validator and register_schema_with_sample. We must find a equivalent mechanism for reading and breaking down a json hierarchy with a delegation process. Such a mechanism must be provided by some library since deserialisation is a common business...

[LorenzoMarnat]

This PR allows to load a tileset and its content in memory by using the TilesetReader class:

reader = TilesetReader()
tileset = reader.read_tileset("path/to/tileset/")

It returns a Tileset instance. It is possible to read the triangles, uvs, material indexes and batch ids from the content of each tile by using the from_glTF method of the TriangleSoup class.

all_tiles = tileset.get_root_tile().get_children()
for tile in all_tiles:
     gltf = tile.get_content().body.glTF
     triangle_soup = TriangleSoup.from_glTF(gltf)

     triangles = triangle_soup.triangles[0]
     vertex_ids = triangle_soup.triangles[1] 
     mat_indexes = triangle_soup.triangles[2]
     uvs = [] if len(triangle_soup.triangles) <= 3 else triangle_soup.triangles[3]

On py3dtilers, there is a Tileset Reader (using py3dtiles' TilesetReader) which can read/write a tileset by keeping:

• [x] Triangles
• [x] UVs
• [x] Materials (with or without textures)
• [x] Batch table data
• [x] Split objects (one geometry per city object)
• [x] Keep child hierarchy

Loading a tileset and its geometries in py3dtilers allows to access the Tilers transformation methods like:

• Rescaling
• Projection in/out
• Translation (on X, Y, Z)

[EricBoix]

This issue was split in three parts

• a first part concerning the reader
• a second part concerning the information display
• a third part concerning a comparison mechanism