store is_translated metadata in minhash

[bluegenes]

6-frame translated sketches are useful for searching protein databases, but also come with a few complications.

• translated signatures should not be compared to each other (containment, jaccard, ANI will likely be incorrect/less useful) Properly document/warning users about ANI b/t translated sequences and protein. #2010
• translated gather --> protein database will report incorrect % classified (how should we adjust output when using 6-frame-translated signatures? #1087)

We could at least keep track of whether or not the sketch was generated via translation in the Minhash object, perhaps as is_translated property (ref #268). This would enable us to warn users about potential issues, or change behavior if desired.

first suggested here: #2010 (comment)