Add a finite volume method for ScalarEq

Proof of principle.

Introduced a (suboptimal) extra FVElement type and added another type parameter to Mesh. Added a ref test for the advection eq with periodic sine wave.

This PR also did some refactoring of the ScalarEq project:

• remove all occurrences of the old broadcast methods (now using broadcast_*_2! or newer)
• introduced some abstractions for Mesh to compactify time step computation

So far only 1d is implemented!

src/ScalarEq/callbacks.jl

-function make_callback(env, P::Project, bdryconds::BoundaryConditions)
-  cbfn_equation(u, t) = callback_equation(u, t, env, P, isperiodic(bdryconds), P.equation, env.mesh)
+function make_callback(env, P::Project, isperiodic::Bool)
+  cbfn_equation(u, t) = callback_equation(u, t, env, P, isperiodic, P.equation, env.mesh)
   cb_equation         = FunctionCallback(cbfn_equation,
                                          CallbackTiming(every_iteration=1,every_dt=0))
-  cbfn_tci(u, t)      = callback_tci(u, t, env, P, isperiodic(bdryconds), P.tci, env.mesh)
+  cbfn_tci(u, t)      = callback_tci(u, t, env, P, isperiodic, P.tci, env.mesh)
   cb_tci              = FunctionCallback(cbfn_tci,
                                          CallbackTiming(every_iteration=1, every_dt=0))
-  cbfn_hrsc(u, t)     = callback_hrsc(u, t, env, P, isperiodic(bdryconds), P.hrsc, env.mesh)
+  cbfn_hrsc(u, t)     = callback_hrsc(u, t, env, P, isperiodic, P.hrsc, env.mesh)
   cb_hrsc             = FunctionCallback(cbfn_hrsc,
                                          CallbackTiming(every_iteration=1,every_dt=0))
   callbackset = CallbackSet(cb_equation, cb_tci, cb_hrsc)
@@ -206,8 +206,8 @@ end
 function callback_tci(state_u, state_t, env, P, isperiodic,
     tci::Union{TCI.Diffusion, TCI.Minmod, TCI.Threshold,
                TCI.ModalDecayAverage, TCI.ModalDecayHighest}, mesh)
-  @unpack cache, mesh            = env
-  @unpack u                      = get_dynamic_variables(cache)
-  @unpack flag                   = get_cell_variables(cache)
+  @unpack cache, mesh = env
+  @unpack u           = get_dynamic_variables(cache)
+  @unpack flag        = get_cell_variables(cache)
   TCI.compute_indicator!(flag, u, tci)
 end