Fixed bugs in averageGraph:
- output of benchmark statistics and pairwise comparison was overlapping
- benchmark mode was broken after introduction of align mode (when no sequence file given we were always creating dummy sequence in both align or benchmark modes)

Improved:
- better handling of modes through mode variable that takes 1 of 3 states
- better input check
- in align mode now calling another GraphAverager constructor instead of creating dummy seq/tag within averageGraph
- changed constructor in GraphAverager: now sequence not a parameter, instead taken from alignment. Check for input moved down to averageGraph
- now allowing alignments that contain other sequences than the target and the templates
- new exception GraphAveragerError thrown in GraphAverager constructor when sequences of alignment and templates don't match