allows opening R in an external terminal when tcsh is the default shell

In tcsh environment variables cannot be set on the command line, and therefore the external terminal fails. 

I'm not aware of a good way to identify the currently running shell so I'm assuming bash as the default
- If indeed the current shell is bash then $BASH should be defined (and the previous behavior is maintained).
- If it is not I fall back to a normal mode where only the desired command is specified. 

In my environment (tcsh is the organization default) this fix works.

Best,
Nir