Check travel_dosimeter selection in analyse_Al2O3C_Measurement()

[RLumSK]

Expected behaviour

According to the manual, the aliquot number has to be selected.

Observed behaviour

The selection works on the basis of the position number, not the aliquot.

Cross-checks

Depending on the selection, the shiny app will be affected as well, i.e. any change has to verify whether the shiny app still does what it should do.