lapack/netlib: add Dtbtrs

[vladimir-ch]

Please take a look.

[vladimir-ch]

I think I still haven't figured out how to update this repo in sync with gonum with modules. I have to run go get -u gonum.org/v1/gonum@master to get rid of the travis failure, right? Is that the expected workflow?

[kortschak]

Because we are actually versioned now, you need to do go get gonum.org/v1/gonum@eea0b5cb5cc92420e0be3a49bbdaeb8aa32d309b (I chose the oldest commit that would support this, but you can use the master sha.

[vladimir-ch]

Ok, done.

[codecov-io]

Codecov Report

Merging #77 into master will decrease coverage by 0.00%.
The diff coverage is 37.50%.

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
- Coverage   29.01%   29.01%   -0.01%     
==========================================
  Files           3        3              
  Lines        6249     6277      +28     
==========================================
+ Hits         1813     1821       +8     
- Misses       4003     4023      +20     
  Partials      433      433              

Impacted Files	Coverage Δ
lapack/netlib/conv.go	100.00% <ø> (ø)
lapack/netlib/lapack.go	29.85% <37.50%> (-0.03%)	⬇️
blas/netlib/blas.go	28.51% <0.00%> (ø)

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