relax SVN archive

Started to implement the read, sequence_exists, and valid_sequence functions.

Small fix for RelaxInvalidSeqError.

Added a new RelaxError for invalid sequence data.

Wrote a unit test for the writing of an amino acid sequence to file.

Added unit tests for all arguments of the sequence.write() user function.

Updated the sequence.write() user function for the new relax design.

Fixed all user function arg unit tests to use the new DATA_TYPES module array.

Alphabetically sorted the initialisation of DATA_TYPES to bring some order to the chaos.

Added the tmpfile() file descriptor to the 'DATA_TYPES' array.

Removed the return_data_types() function and converted data_types into a permanent module constant.

This is in preparation to add file descriptors to the list.