More usage of callbacks using or computing multiple quantities
now do so on scalars because the macros in GSLL pull out the
individual elements of foreign arrays, and accept multiple values and
put them into the right foreign arrays. In roots-multi,
#'multiroot-slot takes a mobject and extracts the mpointer before
calling the foreign slot value. Several examples in
solve-minimize-fit now have optional 'print-steps argument. These
examples are now the basis for additional tests.