Rename counter to size in TBranch UI

[reikdas]

Addresses @HDembinski 's comment - https://github.com/scikit-hep/uproot/pull/477/files#r419114287

@jpivarski should I also rename the corresponding attribute used internally like I did in b022cb9 ? I am a little hesitant because size could mean other things.

There are 3 possibilities in my mind -

1. Leave it as it is (the current state of this PR).
2. Just rename the newbranch class' size attribute (and all references to it) so that users could first create the branch without specifying the size parameter and then do a branch.size = .. to change it.
3. Replace all occurrences of 'counter' in the codebase(including the _iscounter class variable) with 'size'.

Which one do you prefer?

[jpivarski]

Good. Thanks!