openxlsx_getOp() retrieves the "openxlsx" options found in
op.openxlsx. If none are set (currently NULL) retrieves the
default option from op.openxlsx. This will also check that the
intended option is a standard option (listed in op.openxlsx) and
will provide a warning otherwise.
openxlsx_setOp() is a safer way to set an option as it will first
check that the option is a standard option (as above) before setting.