Easy installation of OpenSoT

[EnricoMingo]

We may want to have OpenSoT installable via simple pip or another package manager.

[olivierrochel-inria]

Creating a wheel (pip) is relatively straightforward, but there are some caveats to consider to ensure the package is fully relocatable:

• Finding backends (dlopen at runtime): we must ensure that libopensot can locate them.
• Finding resources: currently they are stored in OPENSOT_RESOURCES_PATH, which is set at compile time (PUBLIC OPENSOT_RESOURCES_PATH="${CMAKE_INSTALL_PREFIX}/share/OpenSoT/resources"). I will add something to allow overriding this path to match where pip installs files.