Add secondary structure prediction.

[shuuul]

Solve the problem in #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]

Codecov Report

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

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

Additional details and impacted files

@@            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	Coverage Δ
src/BioStructures.jl	100.00% <ø> (ø)
src/pdb.jl	88.65% <85.71%> (ø)
src/mmtf.jl	99.33% <96.42%> (ø)
src/spatial.jl	95.11% <97.43%> (ø)
src/model.jl	96.30% <98.48%> (-0.01%)	⬇️
src/mmcif.jl	98.73% <100.00%> (ø)
src/secondary.jl	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?