Load pipelines by name and register them as entry_points

[csala]

Implement pipeline loading by name and use entry points for pipeline discovery.

The API will work as follows:

• The primitives argument will be made a keyword argument instead of a positional argument.
• A new keyword argument pipeline will be added to the MLPipeline class in the first place.
• The pipeline argument will accept four different types:
  • It it is an str it will be considered to be the pipeline name and it will be searched.
  • If it is a list it will be considered to be the primitives list.
  • If it is a dict it will be considered to be a complete pipeline specification
  • If it is an MLPipeline instance it will be converted to a dict using its to_dict method.

For pipeline discovery, the mlpipelines entry_point name will be used like the mlprimitives one is currently being used.
For this, the module primitives will be renamed to discovery and the methods will be remade to work for both pipelines and primitives.