Following the recent changes, kernel data has been split into multipole
and local development data. A kernel also needs some symbolic data about
the tree node it is working on. This data is very generic and should
therefore not be reimplemented each time.

As of this commit, the adaptive tree passes FSymbolicData objects
(introduced by this commit) to kernels; the uniform tree passes the
kernel cell which inherits from FBasicParticle.

The two trees should use the same structure in the future.