Set residue reference SASA through configuration file

[mittinatten]

One should be able to define the reference values for relative accessibilities in a configuration file. Should possibly allow alternative backbone definitions too (but this would require some refactoring). (Related to #27)

[mittinatten]

This requires thinking out a format, writing a new parser and passing it to the classifier. Should not be very difficult, but some work. Looking for volunteers. 🙂