Complex content expressions don't parse in dataframe API

[jleibs]

Example:

view = recording.view(index="timestamp",
                      contents={"/** -/map/ego_vehicle/ego_vehicle/**": ["Scalar", "RotationQuat", "Translation3D"]})

[jleibs]

This is annoying... we've NEVER supported " " as content sub-expression separator. The two forms we currently support are:

list-elements: used in Blueprint content expression API + SpaceViewContents query component.

["/**", "-/map/ego_vehicle/ego_vehicle/**"]

New-line separted: used in selection panel

/**
-/map/ego_vehicle/ego_vehicle/**