Add repeating decimal notation in Calc for representing rational fractions

[rlwhitcomb]

As in 0.(142857) (equal to 1/7).

Would need:

• lexical syntax
• recognition in CalcObjectVisitor
• a way to figure out what the rational fraction represented is
• format notation to use this
• appropriate conversions for calculations
• documentation

There's probably more. So, NOT a trivial issue to resolve.