Alto confidence refactor

[ibenes]

A refactor of ALTO & confidence computation stuff. Consist of 3 chunks of changes:

1. Export of ALTO XML is overall refactored, mainly by extracting auxiliary methods.
2. Export of ALTO XML no longer changes the lines in layout, i.e. it does not alter their confidences and it does not filter lines themselves.
3. The actual computation of line confidence is changed from worst-of-best character probabilities to mean character probabilities.