improve double converting by a pre-compiled regular expression

Current Result

org.zkoss.poi.ss.formula.eval.OperandResolver.fpRegex will be used for many times when converting a string to a double in a formula.

Proposed improvement

As Pattern javadoc recommends:

If a pattern is to be used multiple times, compiling it once and reusing it will be more efficient than invoking this method each time.

We should use a compiled one.