Adjust \partmarks, so that it uses \eqno when within display maths.
This is, I hope, a satisfactory resolution to this long-standing niggle: the
main use for \partmarks is for after equations, and this solution -- using
\eqno to position the indicator -- is probably the most robust, and best
looking. The by-eye alternative is still there, though, but documented as being
for lists, only.
Closes issue #1