In block preconditioners have matrix_pt() get pt from master

* matrix_pt() for block preconditoners always needs to be the full
  Jacobian.

* But setup(...) sets it to the block (in subs block preconditioners),
  so we had to manually change it always

* Instead just get the correct one from the master!

* Might need some renames to make all this clearer
Rename block setup overload in GPBlockPreconditioners for safety

* block_setup is non-virtual so would have to be very careful when
  calling it.

* Renamed it instead since it should only be called from inside the
  preconditioners own setup() anyway.