* structures.ipynb starts to work
   (at least the examples part)
   For some reason, upload button is not displayed
 * enable travis

 * fix versions of pre-commit packages

* Migrate to python 3
Also reemove aiida from the dependency list. This is needed to lock
the dependencies using Pipfile: aiida-core -> circus -> pyzmq <17.0
notebook -> pyzmq>=17
* Apply pre-commit hooks to the code

widgets are imported from the repository:
https://github.com/cpignedoli/aiidalab-empa-surfaces

List of widgets:
1) structure browser widget
2) metadata widget
3) submit button widget

Also remove functions introduced in fbc917f
Since they weren't proven to be useful.

Files added:
1) .pre-commit-config.yaml
2) .prospector.yaml
3) .pylintrc
4) .style.yapf

Python3 compatibility:
1) computers.py: fix utf-8 problems

Code refactoring:
1) Factor out predefine_settings, valid_arguments and put them into utils

Silent some pylint warnings, where appropriate

* New widgets:
  1) ProgressBar
  2) ExportButton
  3) RunningCalcJobOutput

* Fix download function in FolderData and StructureVisualizer widgets

* Replace `aiidalab_display` function with a `viewer` function that returns a
a viewer object able to display the given AiiDA object:
* StructureVisualizer:
  1) Add thumbnail property
  2) Simplify and unify the conversion to cif, xyz and png formats

This parameter defines how many folders one should go up in order
to arrive at the apps' root folder.

Before StructureBrowserWidget was a stand-alone. To unify the access and
simplify the usage I made it insertable into the StructureUploaderWidget.

Add the `ProcessFollower` class that is able to follow a process until it is completed. At the
creation time the `ProcessFollower` accepts a list of followers that can check a particular
outcome of the running process and display it. The followers are specified at the moment of
`ProcessFollower` creation. For the moment we have two followers: `ProgressBarWidget` that
displays the progress and `RunningCalcJobOutputWidget` that displays the output of a running
process.

* Move structure browser widget into structures.py

* Initial implementation of SMILES importer.

Move `SubmitButtonWidget` from a separate file into `process.py` where all process-related widgets are located

As at the current state of things, the metadata widget can be be made general enough, it is better to not provide it as a part of the package.

Since the `StructureUploadWidget` has become a widget of more general-purpose, not only for
uploading - it makes sense to rename it so that the name reflects it's nature. The widget has been
renamed to `StructureManagerWidget`. The only purpose of the widget is to call other single-purpose
widgets to upload structure, select structure from AiiDA database, upload structure from public
databases, etc. Two single-purpose widgets were extracted from old `StructureUploadWidget`: new `StructureUploadWidget` that only allows uploading structures from a computer and `StructureExamplesWidget` that allows providing example structures.

To avoid code duplication I used StructureViewerWidget to display structure
in StructureManagerWidget. The code of the viewer has been improved
thanks to the suggestions of @csadorf.

* Disable smiles widget if openbabel library is not present.

Co-authored-by: Aliaksandr Yakutovich <[email protected]>

* Remove six module imports.
* Remove __future__ imports.
* Remove 'python-modernize' from git pre-commit installs.

* Extract Base parts into a separate class that can be reused. It defines the basic outline
and properties. Also, it adds a configuration panel. Finally, it contains the `selection` trait.  
* Inherit `StructureDataViewer` widget from the base one. It adds `structure` trait.
* Create CopyToClipboardButton widget that has `value` trait used for a copy.

Co-authored-by: Carl Simon Adorf <[email protected]>