#767 Add context method to StackFrame class

[Rony G. Flatscher]

In debug situations it would be very handy to become able to get (read) access to each of the StackFrame's context object. Among other things it would allow to inspect the variables and their values in the context of each StackFrame.

Therefore please add a 'context' method to the StackFrame class that returns the respective RexxContext object.