alias TreeProcessor to Treeprocessor

[Celelibi]

In the interest of consistency with BlockProcessor and most other *Processor. I suggest to provide an alias TreeProcessor to the class Treeprocessor.

This would reduce the friction for extension writers. At least a bit.

[waylan]

The Contributing Guidelines are relevant here:

Legacy code which does not follow the guidelines should only be updated if and when other changes (bug fix, feature addition, etc.) are being made to that section of code. While new features should be given names that follow modern Python naming conventions, existing names should be preserved to avoid backward incompatible changes.

I realize an alias avoids the backward compatibility issue, but personally I'm meh on the whole thing. Of course a PR is always welcome.