Add training and dataset managemnt

[lorenzocerrone]

• New structure for plantseg local files:
  before all plantseg user files were stored in .plantseg_models, and now:

.plantseg/
-------------/configs/
------------/models/
-----------/user_dataset.yaml

• New module dataset_tools to handle operations on datasets
• New plug-ins for managing datasets from napari
• Training as a stand alone widget
• all in Add support for training #159