Base chunkshape of VLArray objects on the number of rows, instead of the total size

[faltet]

When I first implemented the chunkshape size estimation for VLArrays, I based the computation on the fact that all the VLArray was going to be compressed. This turned out not to be the case because only the header for each row is compressed. So I need to update the logic of this calculation.

This will probably imply to deprecate the expectedsizeinMB parameter and add a new one called most probably expectedrows. An strategy for doing this should be devised in order to easy the transition.