Add method "shoulder detection"

In recent experiments >>2020.06.16_Juelich_IEK6H3_Engstelle<< (as well as planed for CroMa-Experiments) shouldermarkers where placed on participants for later extraction with petrack.

right shoulder: blue; left shoulder: green; diameter of markers: 7 cm

This issue should solve the following points:

• discuss location of method in the code / how method should be implemented

• implement method

      - [x] detection of shouldermarkers
      - [x] allocation of shoulders to a head/person
      - [ ] calculation of shoulder orientation (?) ---> decided 07/2021: no
      - [ ] add visualisation of shoulder in PeTrack

• add checkbox in gui for export of shoulders

• add export of shoulderlocation