Implement DmpContextual in v2

[stulp]

Following the "train off-line in Python, execute real-time in C++" approach

[Ekanshh]

Hey @stulp, just wondering if it will be implemented soon or not. LMK :)

[stulp]

A python version is available in another branch (that I will merge into main in a few months):
https://github.com/stulp/dmpbbo/blob/iri/dmpbbo/dmps/DmpContextualTwoStep.py
With a demo here:
https://github.com/stulp/dmpbbo/blob/iri/demos/python/dmps/demo_contextual_dmp.py