Optimise basket sizes created by RootCnvSvc

Previously the basket size configuration could only be coarsely applied to all
branches but in most real world files the size of the data in each branch varies
significantly. Setting a small value causes large branches to be packed into
individual baskets and prevents effective compression. Setting a large value
improves the compression ratio of large branches but causes excessive memory
usage.

Instead we change the parameters to only specifiy the maximum and minimum
branch size and provide a target for the number of events to pack into each
basket. We then use the first event to estimate the size of the branch.

This makes the branching very sensitive to the first event size however this
seems to work okay in practice. If needed the basket sizes could be refined
after several events to give a more precise estimate.