Staggered field access and staggered fields with fluxes to edges/faces

The first index dimension is always used to identify the staggered point, any further ones can be used to store vectors/tensors at these points. f.staggered_access("N") or f.staggered_access(0, sp.Rational(1, 2))) is now supported. The string representation of the resulting accessor is

f_{(0,\frac{1}{2})}

. Furthermore, staggered fields can now have more staggered points than spatial dimensions, i.e. to store fluxes to edge/face neighbors (e.g. f.staggered_access("NE").