Add support for specifying which variety pairs a set of similar segments apply to

[ddaspit]

Currently, any specified similar segments apply to all variety pairs in a Cog project. Some users would like to have more fine-grained control of this, so they can specify which variety pairs a particular set applies to. This would help to deal with outlier varieties in a project.

[ddaspit]

Here is a suggestion I received from a user regarding this issue:

I was just thinking about the Cog feature to define segments as similar. Presently this is a global setting that applies to all variety pairs which might not be appropriate for pairs involving 'extreme' varieties.

Would it be possible to introduce a 'scope' variable for each pair of segments that are defined to be similar? The default would be that the definition applies globally to all variety pairs, but could there be an option to specify a list of pairs for which a particular definition does not apply. [Perhaps this option could be visualised as a lower triangular matrix on which you click the relevant cell to omit a particular variety pair.]