improvements for multiple alts (self, by_alt ops)

v0.2.7

• restore multiple threads per annotation file after fix in biogo/hts (#64)
• new op setid builtin to set the ID field and remove need for lua and fix some bugs.
• fix bug in self with multiple alternates when there were multiple overlaps (thanks Matthew).
• fix for #68, #69 -- with Number=A, op="self", the output will always have a number of elements
  equal to the number of alternates. If there are > 1 annos for a given site, later values will
  overwrite previous.
• new op by_alt that will have Number=A and will append multiple annotations for the same alternate
  (from the same file) and output them as pipe-delimited. e.g with 2 alts, it might look like: 0.111,0.222|0.333
  when the 1st alternate has a single value and the 2nd alternate has 2 values.