use idShort instead of index in idShortPath for elements of SML

[BirgitBoss]

Is your feature request related to a problem? Please describe.
In #432 it was discussed to also allow idShort for elements within a SubmodelElementList. AASd-120 was removed that retricted that there is an idShort.

What is still not possible: usage of idShort within IdShortPath, for IdShortPath index shall be used

Describe the solution you'd like
Check whether idShort should be allowed within IdShortPath