implement FV method for Euler equation (!77)

* update some ref tests

- euler_isentropic_flow
- euler_sod_shock_tube_bernstein
needed because previously we employed a timestep that was too large,
because it only used the local cell width but not the smallest grid
spacing

* add ref test for FV euler sod shock tube

* update ScalarEq so that FV version can use non-periodic bdry conditions

* implement FV method for Euler equation

- also refactor timestep to work with DG and FV elements
- also refactored fv_update_step! methods to now accept buffers for bdry
  values