cognitive fun!

Added comparison tests for the dual n-back variants, and a consonant span task

The comparison tests are added for interest and for completeness. The cognitive load for the single modality version of the dual n-back task is much lower. As the target probability is the same, and the number of trials is also roughly the same, the task would probably get boring very fast. Nevertheless, some people would find it interesting to see how "easy" the single modality test is compared to the dual modality version.

The consonant span test is not, to my knowledge, a standard cognitive test, but it will be a nice segue into reading and lexical processing tasks. This test is made intentionally hard for the phonological loop. Indeed, it would be more effective at discouraging use of the phonological loop if there were auditory distractors, but given the numerous possibilities about this, and that the user could simply just turn off their speakers, there is no auditory distractor in this test.

Oh yeah, clutter is surely becoming a problem now.