add ambiguous partials next to inner nodes

[westram]

When adding partial sequences, it often happens that multiple possible insert positions (PIP) are detected. This happens whenever the full-length sequences (considered for insertion) do not differ in the alignment region which is covered by the partial sequence.

Currently arb warns ("Insertion of '<name>' is ambiguous") and chooses the first ambiguous PIP.

Wanted behavior:

• if all PIPs form one subtree ⇒ insert partial as brother of that subtree
• if the PIPs spread all over the ⇒ reject insertion
• if sth inbetween ⇒ ???