Make ids (e.g. of sceneparts etc) an integer property

[dokempf]

Currently, these can be strings, because in XML everything is a string. After discussion with Hannah, we should however make all of these integers. Also, the ScenePart class should have an id property that can be set on the Python side.

[han16nah]

Let's limit "all of these" a bit. This refers to

• ids of scene parts
• ids of scanner channels

IDs of scanners, platforms, and scenes will remain strings.
@lwiniwar

[lwiniwar]

I'm a bit cautious here, as this introduces inconsistencies with scanner, scene, survey, etc., where we use XML identifiers to cross-reference (e.g. scene="file.xml#scene1"), which would then work differently for a subset of the objects in the helios namespace.
I understand the long-term goal is to overcome XML files, but maybe we can find a cleaner way for the time being?