Improve volumes - for z-stitching

See merge request !118

This reverts commit bdf38b90.

utils: volume: if input and output volumes are HDF5volume instances then use a VirtualLayout to avoid data duplication

Note: Virtual sources are using relative links
fix also saving metadata once concatenation is done

Signed-off-by: Henri Payno <henri.payno@gmail.com>

now those functions expect `index` and `axis` instead of `xy`, `yz` and `xz`

test_get_bounding_box: remove usage of magnification. the pixel size must be the `sample_pixel_size` (aka Voxel size too)