DNA --> nucleotide and protein

[olgabot]

When I first saw sourmash I thought I couldn't use it because I had RNA not DNA. And now I see you do protein signatures too! So I propose this change so others don't get deterred either.

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov-io]

Codecov Report

Merging #473 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #473   +/-   ##
=======================================
  Coverage   90.51%   90.51%           
=======================================
  Files          33       33           
  Lines        4892     4892           
  Branches       36       36           
=======================================
  Hits         4428     4428           
  Misses        463      463           
  Partials        1        1

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