expression_explorer: harcoded Rscipt path

The path to Rscript is hard coded which means that if this binary is in a different paths it will not be found:

/usr/local/bin/Rscript -<<"EOF" ${SCRIPT_DIRECTORY}

I encountered the issue because my local linux installation stores it in /usr/bin/Rscript, a different path on falcon, and if anyone uses conda it is likely it will be somewhere else.

A solution, tested on ubuntu 18.04 is to replace the line this:

$(which Rscript) -<<"EOF" ${SCRIPT_DIRECTORY}

which will find the path to Rscript whichever that might be. Since most users use OSX I am not sure who big of a problem this is, but the bug fix would solve it. Shall I go ahead and make the change?