finish incorporating binomen fxns

[sckott]

via #12

[sckott]

• [x] pick() - pick out one or more taxonomic groups
• [x] pop() - pop out (drop) one or more taxonomic groups
• [x] span() - pick a range between two taxonomic groups (inclusive)
• scatter() - not necessary
• assemble() - not necessary
• strain() - will open separate issue for this, may want to incorporate, may not
• name() - not necessary
• uri() - not necessary
• rank() - not necessary
• id() - not necessary

[sckott]

strain issue opened #108