For now process is always call. When call from workflow_concept it insure the input has benn provided previously. Otherwise you should provide the xas_object. It might evolve with time.

Add some simple unit test to insure an orangecontrib est workflow can be converted to a graph and a graph to an est workflow.
Add test to insure we can execute it as well

Make `Process` inherit from Task
Create a OWESRFWidget which inherit from OWWidget and implement stuff for the task graph.

* _NexusDatasetDef: can now have some units defined by latex: `units_latex`
* _NexusSpectrumDef: can now have a `title` and a `title_latex` defined

In fact instead of having to repeat those definitions n times we should have some king of a database defining those once for all

rename parameter 'data' in 'write_xas_proc' to 'results'

- refactor types to create submodule
- Spectra is now a class instead of a simple list from XasObject
- Spectra contains data which is a numpy ndarray of Spectrum. This was it is simpler
to reshape it andd should offer some flexibility for the futures.
- this is the main PR but several smaller one are expected to come for cleaning...

Avoid creating DataUrl with empty path. Return None instead