Quiet madchap floating-point precision diffs on cygwin.

Numerical accuracy differences across platforms are not entirely surprising. It can depend on numerical library implementation choices as well as compiler heuristics (such as FPU register allocation).  Running these all down and getting the same exact answer on all machines (especially across different compilers/runtime libraries) could be very time-consuming.

The strategy adopted here was to just print out a fixed number of digits.  I also replaced quantities that printed out as "negative zero" with a positive zero, so that small fluctuations around zero do not produce a discernable difference in the output.  The .good files were updated accordingly.  This eliminates the apparent differences between the cygwin and linux outputs, while still displaying enough digits to allow detection of a serious computational error.