set status as string

[lmichel]

For the moment, status are given as integers with a set of allowed values.
After having looked at Simbad TAP output, it turned out that giving status and allowed values as strings would be very useful (e.g. the coordinate quality is given by a letter ranging from A to E).

I suggest to:

• remove the Label class
• remove the Bitfield class for which I never get any concrete use case
• type Status and allowed values as strings. That way the former Label use cases would also be covered .