Add time, space, and I/O complexity to Documentation

[SSoelvsten]

The algorithms in this project are quite complex and their complexities are not at all similar to other BDD packages. Hence, we ought to make the time complexity much clearer to the end user.

• [ ] Add a \complexity command (see here https://www.doxygen.nl/manual/custcmd.html)
• [ ] Annotate the complexity of each BDD and ZDD operation