This prevents ambiguities between `NodePlugin` objects that have been
registered (and are thus instantiable) and those that belong to `Plugin`
objects but have not been registered.
The validation of the node descriptions will be handled directly within
the `NodePlugin` objects and there is thus no need for this method to
exist outside of plugins.py.
A `Plugin` object contains a collection of nodes, represented as
`NodePlugin` objects. This commit adds the implementation of the `Plugin`
class with the methods to add and remove `NodePlugins`, as well as an
empty class for the `NodePlugin` objects themselves.
Add a new module named `plugins` with a `ProcessEnv` class which
contains the paths describing the environment needed for the plugin's
(node's) process.
