Changes to Solver classes

o Multiple solvers can now be created, using separate options

o By default solvers don't save anything to dump file, unless explicitly
  added (solver->outputVars(dump)). This is done for the main solver
  in boutmain.hxx or physicsmodel.hxx
  -> Any additional solvers won't corrupt BOUT.dmp.*

o Restart files can be disabled by setting "enablerestart" to false
  in the solver options.
  -> Any additional solvers won't overwrite BOUT.restart.*

o Base Solver class handles all restart file writing, rather than individual
  solver implementations. This made disabling restart files cleaner.

o Small bug in RK4 cleanup fixed

o Bug in new Field3D yup and ydown implementation fixed. Was
  using mesh->FCI in Field3D constructor, which may be called before
  main().