Also fix bug in description that resulted in a wrong definition of t.
And rename keyagg coefficient from 'mu' to 'a' since we don't use the term "musig
coefficient" anymore and a is what is used in the paper.
Besides reducing the number of arguments, this also removes the R argument from
PartialSigAgg which was not defined precisely:
* The final nonce ''R'' as created during ''Sign'' or ''PartialSigVerify'': a point
Moreover, this paves the way for adding the tweaking, which requires
PartialSigAgg to also have access to challenge e and can now be easily computed
from the Session Context.
We will need more of these explanations and it's better if they do not interfere
the specification section. The remarks section is intended for content that's
not required for implementing the spec.
