Fix parameters for "Design Primers"

[westram]

• primer distance
  • complains if impossible

    invalid minimal primer distance (more than right.max - left.min)
    

    but does not skip primers with invalid distance
  • show distance in result list
  • distance should be "number of bases between both primers" (can't even tell if it calculates sth similar)
• primer positions
  • in order to specify left and right positions in a symmetric way
    ⇒ allow negative "number of bases"
    ⇒ start from position + count bases backwards
  • positions are off-by-one ("bio vs. info"); remove doc