Make it possible to use entry points to register DLite plugins

[jesper-friis]

Currently, when someone creates a package that provides a new data model or plugin, users of that package has to either set environment variables or call one of the following functions:

dlite.storage_path.append(...)         # add search path for new data models
dlite.python_storage_plugin_path(...)  # add search path for new storage plugins written in C
dlite.python_mapping_plugin_path(...)  # add search path for new mapping plugins written in C
dlite.storage_plugin_path(...)         # add search path for new storage plugins written in Python
dlite.mapping_plugin_path(...)         # add search path for new mapping plugins written in Python

An alternative, and in most cases preferable solution, would be to allow packages providing DLite plugins to register them using entry points.

This issue is about implementing entry points covering these cases.