added partial support for abritrary precision calculations

Added the variable $RHWorkingPrecision to set a "WorkingPrecision" for the Package.
So far this setting is only used by IFuns and functions required to solve RHPs,
which solely need Cauchy transforms over contours consisting of lines or rays. 

The overhead to support this functionality is about 2% - 3% CPU Time,
when using MachinePrecision calculation.

I'm not really sure if this should be included in the master branch, because it is only an incomplete implementation and not extensively tested. But maybe it's useful for you.