Add jsonGetReal

[rhinton]

I want to use JSON-for-VHDL for managing configuration information for a DSP-heavy design. That means I need to store and read real values. Please add a jsonGetReal function. Probably the fixed-point and/or floating-point packages already have the necessary numeric parsing code.

[Paebbels]

@rhinton what time frame do I have to add this feature?
Are you using it with latest Vivado, where string performance improved a lot?

I'm currently bound to the development of topics in EDA².