(s2gpp) Add Series2Graph++ algorithm

[SebastianSchmidl]

Add Series2Graph++ algorithm to this repository.

• Source code: https://github.com/HPI-Information-Systems/S2Gpp

Either build pip-Package for S2G++, publish it to PyPI, and just use this package in an algorithm folder; or
add repository as gitmodule, and build S2G++ from scratch.