Make PCA analysis of multiple secondary structures a choosable option even when only 1, 2 or 3 sequences are in the alignment or are selected

When choosing Calculate -> Calculate Tree, PCA or PaSiMap... the option to perform a Principal Component Analysis is greyed out when fewer than four sequences are selected or there are fewer than four sequences in the alignment.
When performing a PCA on the secondary structures this should be a choosable option when there are four or more secondary structures.
It would be consistent with JAL-4521 to enable this option when one or more sequence is chosen, and issue an instructive error message when not enough "sequences" (or secondary structures as appropriate) are available, like those added in JAL-4521.

Note there is also an ArrayOutOfBoundException bug in the calculation of secondary structure PCA which is documented separately in issue JAL-4624 since these are two separate matters/fixes.