use "ratio" instead of "rational" for more clarity in some namings. more descriptive error message from write-ratio-as-floating-point.

Change-Id: Ia432e58ca0708dc9444fcacf87bae72c0cc37382
*silently-truncate-ratios* defaults to nil

otherwise the symtomps of the silent data distortion errors
can be *far* away from the time/space coordinates of the
actual error. (which makes debugging much harder than necessary...)