2D grid scans shouldn't care about Y steps being odd

After https://github.com/DiamondLightSource/hyperion/issues/1118, the `xray_centre_plan` throws an error if it is given an odd number of Y steps. This ensures our 3D grid scans work properly, but the change means that this check also occurs for 2D grid scans when it doesn't need to be.
For example, VMXm use 2D grid scans without using the grid detect plan, so the above change will cause their scans to break.

We need to decide how to refactor the 2D / 3D grid scans. We could think of a 3D scan as 2x 2D grid scans, but the motion script doesn't currently work this way