Ghias et al., JCP 225(1), 2007, with image point interpolation
based on R. Franke, Mathematics of Computation 38(157), 1982.
Image points currently have the same distance from IB surface
as ghost points. In this commit, only Dirichlet boundary
conditions supported! Includes test case for flow over 2D hill
("Witch of Agnesi"/"Agnesi hill") based on Lundquist et al.,
Month. Meterol. Rev. 140(12), 2012,
http://journals.ametsoc.org/doi/10.1175/MWR-D-11-00311.1

Notes:
- common_ops scatter/gather is incompatible with lists, therefore
  commented out dim-check.
- To run Witch of Agnesi test case, be sure to initialize the
  Coriolis force as follows:
    _DEFAULT_LAT = 0.5497607357 # rad == 31.5 deg
    # Coriolis force coeff of 1e-4 rad/s from Lundquist et al., Month. Meterol.
    # Rev. 2010 for validation of IB methods corresponds to a latitude
    # defined by _REF_LAT
    _REF_LAT = 0.7555285998 # rad == 43.289 deg
    my_lat = _REF_LAT
    logging.warn(f'@@@ Using reference latitude of {my_lat} rad '
                 f'corresonding to {np.rad2deg(my_lat):.3f} deg.')
    self.coriolis_force_fn = cloud_utils.coriolis_force(my_lat, {
        'u': self.fire_utils.u_init,
        'v': self.fire_utils.v_init,
        'w': 0.0
    }, 2)

added Neumann boundary condition for scalars, and reformatted
code.