insert_scattering_quantity check valid arrays and xtypes

[yucongalicechen]

Problem

Proposed solution

In the insert_scattering_quantity function, we want to add these features: (1) check if xarray is empty or not, if not check that yarray has the same length as xarray and xtype is valid (one of XQUANTITIES), (2) set attribute so that we don't allow setting an invalid xtype.

[bobleesj]

@yucongalicechen

For (2) could you clarify what "attribute" here means?

[yucongalicechen]

@yucongalicechen

For (2) could you clarify what "attribute" here means?

Yeah, it's the similar idea from #194. Use @Property to raise a value error if xtype is not one of the XQUANTITIES.

[bobleesj]

Thanks! I understand it now - using a property decorator