Simple symmetry with some blocks are forbidden.

[xichuang]

Hi, I realize that your excellent library is also suitable for implementing parallel tensor network states(TNS) algorithms. In symmetric TNS, however, the symmetry is a little different from the three symmetry types you have been implemented, and is much simpler. They are just block sparse tensors. I wonder if you can implement a symmetry that can manually mark some blocks as forbidden, i.e., by marking Tij's block with index [1,1] as forbidden.

If it is not listed in your TODO list, any suggestions that I can implement it myself with fewer efforts? Thank you.