[refactor] make s2p a python package that can be installed with pip

[carlodef]

This pull request reorganises the sources in order to make s2p installable with pip. The main changes are:

• s2p is now a python package (instead of a python module)
• the folder s2plib was renamed to s2p
• the file s2p.py has disappeared and its content is split among s2p/__init__.py (most of the code) and s2p/cli.py (command line parsing code)
• when one installs s2p with pip, a CLI tool named s2p becomes available in the system PATH
• the file s2p_test.py was moved to tests/test_s2p.py
• the folder testdata was moved to tests/data
• the README has been updated with simpler install instructions and CLI usage examples

Python codes that depend on s2p should not be affected: import s2p still works in the same way.

Shell scripts running python /path/to/your/clone/of/s2p/s2p.py should be updated to run instead s2p as a standalone command line tool.

This pull request contains the changes of pull request #191.

EDIT: A source distribution of s2p corresponding to this branch's version is available on PyPI and can be installed with pip:

pip install s2p

[jmichel-otb]

I think there should be only a few minor conflicts with #196:

• Name of the data dir
• Imports (unlikely)
• Calls to read_config_file and main in s2p.py

[jmichel-otb]

I just did a merge by hand with #196 and it is fairly easy. There are a few minor conflicts on .travis.yaml and imports needs to be changed from s2plib to s2p in tests folder.