Add viewer

[pruliere]

1 - Add a results viewer using a graphic interface with pyqt:

• viewer can be launched with "python -m fedoo.viewer" or with a python script with fedoo.viewer(data)
• manage several result windows with editable title names. The views can be linked between windows and the plot options can be applied to all windows or only the active one.
• field, component, data_type and iteraction selectors.
• tool to define a line (by picking point) and plot the current results over the line.
• tool to define a clipping plane and clip the current mesh.
• many plot, scalarbar and rendering options can be configurable within the application.
• interactive node and element selectors to easily get some node or element data.
• option to save fig and write movie with the current options inside the viewer.

2 - Improve the DataSet object:

• add clip_args argument to the plot method to clip the plot.
• add element_set and element_set_invert arguments to the plot method to plot only a set of elements.
• plotting quadratic element mesh show only real edges.
• Add copy and deepcopy methods to DataSet objects.

3 – Simplify MultiFrameDataSet plot and write_movie methods:

• MultiFrameDataSet.plot method is now completely inherited from DataSet.
• MultiFrameDataSet.write_movie now make heavy use of DataSet.plot for more options and easy updates. Keep compatibility with the previous write_movie implementation.

4 – Improve axisymmetric reconstruction:

• The function axi_to_3d has been improved. Now use a wrapper to the MultiFrameDataSet object that is memory efficient (convert data on the fly and not all data) and is compatible with the viewer.
• add a fedoo.mesh.axisymmetric_extrusion function.

Remain to do:

• add an element_set tool to the viewer in order to create element sets and hide/show some part of the mesh.
• update the documentation and add an example to illustrate principal plotting options.
• for another pull request: allow to deal with several mesh and data coming from an AssemblySum object.