Modified the R download script so that it uses the Python download mechanism.
Modified the R download script so that it uses the Python download mechanism.

Instead of re-implementing the download script in R we use the rPython
package to invoke the Python functionality. Easier code maintenance.

InsightSoftwareConsortium/SimpleITK-Notebooks,thewtex/SimpleITK-Notebooks,zivy/SimpleITK-Notebooks,zivy/SimpleITK-Notebooks,InsightSoftwareConsortium/SimpleITK-Notebooks,zivy/SimpleITK-Notebooks,InsightSoftwareConsortium/SimpleITK-Notebooks,thewtex/SimpleITK-Notebooks,thewtex/SimpleITK-Notebooks