Add support for decimal data type

[chrishamm]

DSF API only exposes float (single precision) data at present. This needs to be enhanced to support .NET decimal values by default, which will also allow display of an arbitrary number of decimal digits. Support for this needs to be implemented in RRF, too.

[dc42]

Why?

[chrishamm]

To fix #187. Also, when you parse e.g. 12.000 and convert it back to a string, 12.000 is output instead of 12. If you want to output a specific number of decimal points, that's the best way to go.

[dc42]

When RRF stores a float in an ExpressionValue, the value already includes a number of significant decimal places. I think all we need to do is to have that number returned to DSF along with the float value, whenever EvaluateExpression returns a value to DSF. That requires a change to DataTransfer::WriteEvaluationResult.