make all non-implementation headers includable without
preconditions. Also, this removes the GravityColumnSolver.hpp file,
because it tried to include a non-existing file and it was thus unused.
our policy is that we only use boost if necessary, i.e., if the oldest
supported compiler does not support a given feature but boost
does. since we recently switched to GCC 4.4 or newer, std::shared_ptr
is available unconditionally.
