Added dtbsv and tests. Fixed bug in flatten banded and added tests
removed outdated comment
Added future test against dense.

Added test to compare banded with dense. Uncomment when Dtrsv is fixed.
reformatted example matrices
Added TODO comment for dtbsv test
Added back in comments and removed dead code
Made comments full english.
Changed formatting in banded comment to align the numbers on the units column
improved comment formatting
Merge pull request #31 from gonum/adddtbsv3

Adddtbsv3