Support for 64-bit int and real valued Options

Add Int64Option and FloatOption to the options machinery.
Add RealOption that aliases either DoubleOption or FloatOption,
depending on GMX_DOUBLE.
With these additions, the options machinery supports all the
functionality and types of the old t_pargs machinery.

Change-Id: I2260e52d3a220f824e1c3e790da2dba83f381972