The choice of having neither RPA nor IPA corresponds to full response.
It was incorrectly removed.

Default file format for vext and response is XML.
Use -cube option for cube format.

ExternalPotential uses a Function3d object to read a vext XML file.
ResponseCmd option modified: -io not required, default xml.
-nx,-ny,-nz not used, size read from either cube or xml file.

Added IPA to response command. Made slight modification to vext variable to facilitate cDFT implementaion.