Add secondary structure prediction.

[shuuul]

Solve the problem in https://github.com/BioJulia/BioStructures.jl/issues/33

• Add ss_code field to Residue.
• Add run_dssp and run_stride.
• Add support for ProteinStructure with multiple models.
• Add ss_code! to assign ss_code to residue.
• Add ss_code to read ss_code of residue/atom.
• Add helixselector, sheetselector, coilselector

[codecov[bot]]

Codecov Report

Patch coverage: 96.88% and project coverage change: +0.18% :tada:

Comparison is base (bc70e73) 95.51% compared to head (188fe9c) 95.70%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #42 +/- ## ========================================== + Coverage 95.51% 95.70% +0.18% ========================================== Files 6 7 +1 Lines 1695 1769 +74 ========================================== + Hits 1619 1693 +74 Misses 76 76 ``` | [Files Changed](https://app.codecov.io/gh/BioJulia/BioStructures.jl/pull/42?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia) | Coverage Δ | | |---|---|---| | [src/BioStructures.jl](https://app.codecov.io/gh/BioJulia/BioStructures.jl/pull/42?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia#diff-c3JjL0Jpb1N0cnVjdHVyZXMuamw=) | `100.00% <ø> (ø)` | | | [src/pdb.jl](https://app.codecov.io/gh/BioJulia/BioStructures.jl/pull/42?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia#diff-c3JjL3BkYi5qbA==) | `88.65% <85.71%> (ø)` | | | [src/mmtf.jl](https://app.codecov.io/gh/BioJulia/BioStructures.jl/pull/42?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia#diff-c3JjL21tdGYuamw=) | `99.33% <96.42%> (ø)` | | | [src/spatial.jl](https://app.codecov.io/gh/BioJulia/BioStructures.jl/pull/42?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia#diff-c3JjL3NwYXRpYWwuamw=) | `95.11% <97.43%> (ø)` | | | [src/model.jl](https://app.codecov.io/gh/BioJulia/BioStructures.jl/pull/42?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia#diff-c3JjL21vZGVsLmps) | `96.30% <98.48%> (-0.01%)` | :arrow_down: | | [src/mmcif.jl](https://app.codecov.io/gh/BioJulia/BioStructures.jl/pull/42?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia#diff-c3JjL21tY2lmLmps) | `98.73% <100.00%> (ø)` | | | [src/secondary.jl](https://app.codecov.io/gh/BioJulia/BioStructures.jl/pull/42?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia#diff-c3JjL3NlY29uZGFyeS5qbA==) | `100.00% <100.00%> (ø)` | |

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[jgreener64]

Thanks for doing this. Could you undo the general formatting changes so that I can see the line diffs better?