Modified the internal formatting of the data section of the Grace 2D graph files.
This affects the lib.plotting.grace.write_xy_data() function. The formatting is now more
consistent, with the X value now set to a fixed number of decimal places, and hence will no longer
change between Python 2 and 3. The data is now all right justified as well, for easier reading.
All affected system tests have been updated for the new format.