relax SVN archive

Fix for the generic_fns.sequence.read() function.

Converted the strip class method to a module function.

Added the missing import of relax_io.strip.

Fixed the path in the unit test for reading a protein sequence.

Updated the extract_data() for the new relax design (converted from a class method to a function).

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.