BUG: linalg.inner handled ndim incorrectly
ENH: Implement general inversion function

This inversion function supports broadcasting and inversion of scalars
(ndim=0), matrices (ndim=1), and tensors (ndim>1).
ENH: Add function for making a set of arrays equal ndim
ENH: Implement general sum-product function for Node

This function computes the product of a set of arrays, sums over the
plates that should not be in the result and applies correction term if
some plates are broadcasted.
TST: Add unit tests for the sum-product function in Node
BUG: Fix bugs in the automatic moment converter

The automatic Moments conversion system had a few bugs:
 * if a converter was added using add_converter method, it was added to
   the same dictionary used by all classes thus all classes used it

Now, each class may have its own list of converters and the parents'
list of converters are also checked.