Extend dssp implementation to also support the promotif standard

[josemduarte]

Now that the dssp algorithm for secondary structure assignment is implemented in biojava (see #336), it would be nice to also have the promotif one that extends it slightly. Ideally we should be able to support both traditional dssp and promotif.

[Shubhangi2901]

Hi @josemduarte I would love to work on this new feature!

[josemduarte]

Sounds good @Shubhangi2901 , we are always happy to get help.

FYI there's a secondary structure tutorial: https://github.com/biojava/biojava-tutorial/blob/master/structure/secstruc.md

The full implementation and tutorial was done by @lafita