Zoom Init test fixed and improved

This branch started simply fixing testZoomInit. It does this by:

• Initialising the mock space with bzero.
• Setting with_zoom_region which was left unset after changing from a parameter to a runtime argument in the main code.
• Ensuring the engine is set to NULL to stop the test looking for gravity properties that don't exist.
• Using a different zoom region extent for each zoom region definition to be more true to their actual use (i.e. small, large and intermediate zoom region sizes relative to the parent box).

I also expanded the test to ensure the code was behaving as expected w.r.t cell grid alignment (also done on a downstream branch in a debugging check). In another win for unit testing, this immediately highlighted that the test constructed an edge case where the zoom and buffer cells were offset by half a buffer cell. Thus this MR also ensures the buffer cells and zoom cells align by ensuring the buffer cdim is odd before applying region_buffer_ratio. The zoom region boundaries are set to be exactly buffer_width * region_buffer_ratio / 2 away from the centre of the box (explicitly what is expected for them to align).

I also did some light cleaning in zoom_region_init while I implemented the above fix:

• The different zoom_get_cell_props... functions now don't take arguments since they were unnecessary in the grand scheme of things.
• The zoom dim is now checked for all methods instead of just the buffer cell method for safety's sake.
• The buffer bounds are reported in the zoom properties report as a sanity check.
• The strange passing of a pointer to a double for max_dim has been removed.
• The check of whether zoom region padding had been increased too far previously wasn't quite right, it errored for any padding >2 rather than checking the user-specified padding hadn't been increased too far. This has now been fixed to compare the user-specified padding factor with the true padding factor found after computing the zoom region properties.