[WIP] Group demo files

[jbarnoud]

Implement my comment in issue #97

This pull request

• moves all demo files in pbxplore.demo, except the files that are in demo_paper
• creates a pbxplore.demo module with a DEMO_DATA_PATH constant pointing to the demo files
• creates a pbxplore.DEMO_DATA_PATH shortcut
• documents all the above
• creates a PBdata program that prints the path to the data files, or list all data files

The help message for PBdata is copied bellow:

$PBdata -h
usage: PBdata [-h] [--list] [--list-abs]

Print info on the demonstration files bundled with PBxplore

optional arguments:
  -h, --help        show this help message and exit
  --list, --ls, -l  List the available files instead of the directory path.
  --list-abs, -L    List the absolute path to the available files instead of
                    the directory path.

TODO:

• [x] fix the demo to get the files from the new location
• [x] document PBdata somewhere

This PR is based on #96. I will rebase the branch when #96 will actually be merged.

[jbarnoud]

I added a page in the doc directory about the demonstrations and the demo files. Some of these files are are related to papers, it would be nice to cite these papers in the documentation. Yet, I do not know what file comes with what paper. @pierrepo ?

Beside that, the PR is almost ready. It just requires an external eye, and #96 to be merged.

Also the name PBdata may not be the best. I ruled out PBdemo to avoid confusion with the demo scripts, and PBdemo-data-files because it is way to long. PBdata may be just fine, but maybe you have other ideas.

[pierrepo]

@jbarnoud OK for the reference papers. How exactly do you want us to cite the references? In the doc string? In a separate file?