Add T-value option for collocations

[lukavdplas]

Adds a new option for calculating collocation scores based on the T-value. Also significantly expands the documentation for the neighbouring words graph.

T-values seem to work in providing a more "balanced" score, compared to the old formula. Results seem reasonable to me but I'll let the people & parliament team test it after release.

Some comments:

• An issue with these formulas is that we have to take a lot of shortcuts with the statistics (now also documented in the manual), e.g. taking background frequencies from the entire corpus instead of the query set. So the validity is... questionable.
• The t-value formula is also available in n-grams mode, but whether that is really valid, I don't know.