sensor attributes are redundant

[geneorama]

@vforgione I didn't catch what you were saying earlier, I see now that the attribute columns are totally unnecessary for sensors because there's a column that has the exact same information.

In R I would say that people rarely use attributes, but they're kind of useful and it's the right way to capture certain data.

I think it might be more efficient to return things like this as a list. Using this paradigm would break anyone depending on the existing code, but now's the time to make changes like that. Technically removing the attributes could also break people's workflow.

[geneorama]

Also, these attributes are all NULL

  attr(df, "subsystem") <- data$subsystem
  attr(df, "sensor") <- data$sensor
  attr(df, "parameter") <- data$parameter