MiniSEED Activity Flag implementation

Hello,

I'm trying to generate a miniSEED file with custom 'Activity Flags' and 'Data quality flags' (as defined in SEED Manual 2.4 chapter 8 page 108). I also need a blockette 1001 with 'Timing quality' and 'Frame count'.

Looking at obspy.mseed.core.writeMSEED, I found no way to add these informations into a mseed file. It seems that reading a mseed file into a obspy.core.Trace discard them as well.

Is there a way to add the 'Activity Flags', 'Data quality flags' and blockette 1001's 'Timing quality' and 'Frame count'? Do I need to create a Pull Request to add support for these values?

Thank you for your support to a beginner to obspy

SF.
Any reason for importing `_struct` and not `struct`?