Add scatter/gather unit tests for vector fields
Prior to 71fbd0f5, the implementation of scatter and gather had a mistake with the types that went unnoticed despite turning the interpolation weights into matrices instead of vectors when vector fields were used. It seems that the feature was tested with only with scalar fields. The issue was unknowingly fixed in !201 (merged).
Expand the PIC unit tests to also verify scatter/gather behavior for vector fields to ensure that such mistakes are more easily caught in the future.