add rotational symmetry mod, dtwavexfm2b and dtwaveifm2b

Originally based on PR #16 by @scf32. Pretty much a line-by-line copy of 
dtwavexfm2b() as it appears in the MATLAB toolbox. You may prefer to merge
dtwavexfm2() and dtwavexfm2b() into one function which can handle the extra
coefficients returned by qshift and biort. As it is, the use of 'qshift_b_bp'
and 'near_sym_b_bp' (which I have included in .npz format) is pretty much
hardcoded in dtwavexfm2b.