Add support to MiniSEED sequence count

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
Not sure to know what test I'm missing. In both the kwarg argument and the trace option, I check 1) whether the user somehow filled the sequence_number value, 2) the type by converting `sequence_number` to `int`, raising TypeError if it's wrong, and 3) the value that must be between 1 and 999999. The test is done at lines 504 to 516 for kwarg argument, and at lines 555 to 566 for the trace. Which test am I missing for the kwarg argument?