Merge mseed files

Hello,

I have to create one miniSEED file from data that may have gaps in it. The best way I've found to achieve this is to use obspy to create one MiniSEED files for each part of continuous data, and then merge them to have one single file.

Therefore I need a method to merge MiniSEED files. Additional treatment is needed to rewrite the sequence number of the records, since each input file starts over at 1.

I haven't seen anything about it in obspy (maybe I've missed it?) but I can code it. My question is: would it have a sense to add it to obspy? If no, I'll just create it for myself. Otherwise, I'll make a pull request to obspy. In that case, I will add support for multi-traces MiniSEED files (probably sorting them by trace and by date).

Cheers,

SF
I think this whole `if sequence_number is not None` block can just be replaced by this single condition. It is ok to let the ValueError surface if it cannot be converted to an integer. Also one can do

```python
if 1 <= sequence_number <= 99999:
```

but that is really just syntactic sugar ;-)