Don't use introspection in MVAComputer

MVAComputer was using introspection to clone an object.  In ROOT 6, this causes unnecessary header parsing, which increases memory use.  It is also significantly slower, in ROOT 5 as well as ROOT 6.
It is easy to do this without introspection.
This pull request is for ROOT 6.  The same change will be done for ROOT 5 (in another pull request) for code commonality, unless it causes problems in ROOT 5.
Please merge this request as soon as convenient.