Change similarity value for non-comparable GO-term/GO-term-sets from -1 to nan

[rababerladuseladim]

Fix #9

Rel_Metric returns float('nan') if

• one of the go terms is not in the got term tree (goatools godag)
• go terms are from different namespaces

BMA function for averaging of multiple similary values returns float('nan') if

• all individual GO term comparisons returned float('nan')
• one GO term set is empty, the other is not