Csv sensor

[Marco-Herrmann]

Added class Sensor
Applied to example with rigid body --> double_pendulum.py
Applied to example with rod --> rod_export_demo.py

[JonasBreuling]

Sieht gut aus. Ich habe es aber nicht getestet.

@lisaeb wie sieht das für dich aus? Was brauchst du zusätzlich/ was machst du anders?

[JonasBreuling]

Please remove the *.csv files from the commit.

[JonasBreuling]

Looks fine to me. @lisaeb what do you think?

[lisaeb]

Looks fine for me, as a default export for now totally sufficient. Everything beyond is solver and/or contribution specific and would require the option to add own functions that are evaluated and added, which we should consider imo in the future. Otherwise, as soon as the need for specialised output arises, say for instance a force, the workflow will become messy/hacky.