rootio: check validity of chaining trees together

[sbinet]

the rootio.Chain function does not check that it's valid to chain 2 (or more) Trees together.

e.g. even if 2 trees do not have a valid intersection of branches (names+types) it will be happy to chain them, and (hopefuly) fail in some obvious (again, hopefully) way.

we should make sure that some reasonable amount of checks is in place.

open question: should rootio.Chain panic or return an error?