SpanFocusQuery() needs to respect classes outside the sub span

[Akron]

Follow up to https://github.com/KorAP/Krill/issues/7 :

Currently focus queries require that classes are part of sub spans. Unfortunately there are cases where this is not true.

Example: The span <a>...{1:...}...{2:...}...</a> is modified using focus(2:...), but still contains a class 1. Now, if the span is again modified using focus(1:...) a preceding match span may be <a>...{2:...}...{1:...}...</a>, so the second class 1 may precede the first class 1.

[Akron]

A naive approach would simply remove classes from payloads that are outside the scope of the current span in the focus query. However, there are probably valid queries that require intact classes and multiple nested focus queries that wouldn't work anymore (e.g. reference queries).

[Akron]

A better approach may be to let the focus query keep track of the largest span (by adding a payload with a fixed class number > 128 including minimal start and maximum end position), and taking this into account (in case it's set) instead of the current wrapped query (see https://github.com/KorAP/Krill/issues/7) when comparing with the highest priority matches.

[Akron]

An optimization to this approach would involve an attribute like "keepTrack", that advises a focus query to alter the payload in case it is wrapped by another focus query. This could be done in the optimization toQuery() phase.

[margaretha]

A better approach may be to let the focus query keep track of the largest span (by adding a payload

It sounds good and quite simple!