add support for SEG2 data format code 2
Adding support for SEG2 data format code 1.

If data format code 2 is working as tested with a received data file
then I am confident that code 1 works as well. Both are just two's
complement integers with unspecified endianness - native byte order is
assumed.
Only data format code 3 is not supported now. It is an interesting
20-bit floating point format so we would definitely need a test file for
it.
adding test data and very basic read test for SEG2 data format code 2, thanks to Simon Kremers for the data example!
update CHANGELOG
seg2:

 - warn on encountering a DELAY header field
 - map DESCALING_FACTOR to stats.calib
Merge branch 'seg2_data_format_2'

Conflicts:
	CHANGELOG.txt
seg2: removing writeSEG2 stub (only raises) and plugin in setup.py to avoid SEG2 getting listed in possible write formats on unrecognized format specified in write operation
seg2: showing warning regarding possible problems with custom headers
seg2: adding file headers to each trace's stats.seg2