Added perSequenceData attribute to DataType so that a summary of
the auxiliary data can be displayed on the Sequences (InputGUI) panel.
This also means that when a sequence is removed from the sequence list
via the GUI interface, all associated auxiliary data is removed as well,
according to the actions specified in the
DataType's removeDataAssociatedWith(String sequenceName) function.

Previously, removing a sequence would cause a mismatch between sequences
and per-sequence data such as ProteinSkeleton's, since there was no way
to remove elements of the latter via the GUI, but the aforementioned
changes fix this behaviour.
Merge branch 'structalign' of ssh://github.com/statalign/statalign into structalign