GSE2 replacing fgets calls to function pointer

* this will allow direct reading of string buffers from python
GSE2: Adding python callback for buffer reading

* also added test on null pointer, could not avoid to the printing of an
  error to stdout
adding decomp_6b_buffer to windows def file
Merge branch 'master' into gse_fct_pointer
function pointer for gse2 compress_6b
making gse_functions.c independent of buf.c
no call to buf.c from python any more

memory is all allocated from python via callbacks
making gse2 StringIO compatible

* removing last use of file pointers
* more direct use of obspy headers, avoiding multiple copies
* new StringIO test
* adapting tests correspondingly
more memory efficient writing
cleaning up

* removing unused functions in windows def file
* removing now unused c_file_p
* simplifying read and tests
extracting compress_CM6 function
Merge branch 'master' into gse_fct_pointer
pep8
bugfixing traces containing less than 80 samples

correcting times in stream.py
updating "corrupt" station name in scan test
Merge pull request #653 from mbyt/gse_fct_pointer

StringIO Compatible GSE2 Support