src/grind.lisp: Adjust MSIZE-ATOM for noun atoms. This fixes a bug introduced
in most recent revision (r1.16) in which nouns are printed with superfluous ?%.
Also cut out special-case for %derivative (it is now handled by code for all nouns).
tests/rtest16.mac: new tests to test recent changes to MSIZE-ATOM via Maxima
string function (which calls MSIZE and therefore MSIZE-ATOM).