Split providers into protocols, formatters and symbolizers

[Jeremy-Gaillard]

Follow up to this comment.

Providers are presently in charge of handling a protocol, as well as generating and styling geometries.

These tasks should be split in several modules to improve factorisation:

• Providers: they fetch data following a specific protocol
• Formatters: they transform the raw data given by the providers into a generic TypedArray (when possible)
• Symbolizers: they create Three.js objects from the generic TypedArrays and styling rules

TODO list:

• Move and rename JS files in charge of formatting into a Parser directory
• Homogeneize the interface of parsers to a single static function : parse(file, options) -> promise
• Move formatting and symbolizing code out of providers
• Move symbolizing code out of formatters
• Create a generic symbolizer module supporting:
  • 3D Tiles styling
  • Vector Tile styling
  • Additional styling from PLU2PLUS

[gchoqueux]

Now the itowns architecture is

• Layer to add data on viewer, with process to update data.
• Source to define data access
• the chain fetch/parse/convert to provide data to viewer