Merge logical systems

[nsbgn]

Since most logical systems tend to be extensions to 'basic' logical systems, we would like to avoid code duplication by building upon its basis, instead of repeating the same rules and axioms for every variant.

There are two obvious (not mutually exclusive choices) for this:

• Allow an include key in the YAML to import other files.
• Allow additive specification of partial system definitions, as in: judge K D 4 more/system.yml

As of now, the latter has my preference, but they are not mutually exclusive.

Implement #2 first.