Merge secstruc library by Tomasz Puton

[krother]

Tomek indicated it might be a good idea to merge his secstruc code with ModeRNA. It provides a lot of functions to work with secondary structures including I/O. The library is in very good shape and has automatic tests. See https://bitbucket.org/tomasz_puton/secstruc

[tomaszputon]

good idea, let me know when it's time to start thinking about what functionality of secstruc should be transfered to ModeRNA