pre-v00-13-01 podio ROOT files can no longer be read with the RootReader

[vvolkl]

I only realize this now, but the changes introduced in #197 mean that old podio root files can no longer be read.

~/r/podio/build/tests (master)$ ./read
Error in <TTree::SetBranchAddress>: unknown branch -> CollectionTypeInfo
reading event 0
terminate called after throwing an instance of 'std::runtime_error'
  what():  No collection 'strings' is present in the EventStore
Aborted (core dumped)

This relates to the metadata discussion #86 but in this case concerns a change in how podio stores metadata on disk.

I think in this case it is worth to try to do some gymnastics to keep the backwards incompatibility - breaking existing data is even more to be avoided than breaking user interfaces.