BUG: writeFile: Increased the number of value characters

The number of characters needed to print a double in scientific format
is 8 plus the number of decimal places; e.g., -6.453452e-231 (6 decimal
places, 14 characters). This has been set in writeFile.C, replacing a
value of 7. Presumably, the case of three digits in the exponent was not
considered when this was first implemented. This change ensures at least
one character of whitespace between tabulated numbers.

This resolves bug report https://bugs.openfoam.org/view.php?id=2801