…ronment configuration

- add option to control verbosity
- in extern mode: disable logging and enable it only for the node
computation (to avoid polluting the node's log with general warnings)

…oom_compute

Reoganization
- BaseNode: is the base class for all nodes
- Node: is now dedicated to python nodes, with the implentation directly
in the process function
- CommandLineNode: dedicated to generate and run external command lines

Avoid qml errors when accessing non-existing nodes.
The entry exist in the dict and we retrieve a null node.

And add the check about duplicates for canBeStopped/canBeCanceled.

When the compatibility node was fully computed, the recompute/resubmit
button was available in the UI menu.
The reason is that we need to propagate that the connected nodes can be
computed if there is a compatibility node in the dependency iff it is
already fully computed.
Now there is an additional explicit check.

The states of nodes were not valid the first time they were loaded, but
became valid after any topological update (such as creating or removing
a node or edge).
The reason was that the loading of the graph was done before setting the
project path (and thus the cacheDir). So the status files of the nodes
were not available during the graph creation and were not updated at the
end of the load.
Now we set the filepath/cache first, so everything is right from the
start.