persistent-test double precision

These two doubles:
x = 1234.56789012345 :: Double
y = 1234.56789012346 :: Double

Are 'the same' with a 14 digit precision, but the test:
```abs (dataTypeTableDouble x - dataTypeTableDouble y) > 1e-14```

Will return true nevertheless.

The provided alternative first converts each operand to a common format (1.23456789012345), so that the difference comparison makes sense. Of course, my math may be off, so feel free to check it!