Using C++ interface in python

I think python should use the C++ interface rather than the C one.
The reason is that the C++ interface included in PLUMED 2.5
rethrows exceptions and thus can be used to load
a shared library compiled with a different compiler or
linked against a different standard library.

In perspective, this will allow compiling PLUMED with
a different compiler/library than that used to compile
python itself.