More callbacks take and return scalars
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.
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