Two-Way ZDD!

[gabeschoenbach]

The majority of this PR involves adding two_way_zdd.jl, which creates the following functions:

• construct_halfway_zdd() which is identical to construct_zdd() except it returns the list of Nodes at precisely the halfway point along the ZDD
• number_compatible() which takes the two lists of nodes as given by construct_halfway_zdd() and some other arguments, and uses helper functions in two_way_zdd.jl to calculate the number of compatible pairs of "forward" and "backward" nodes. This number should be equal to the number of solutions to the whole ZDD.

This PR amends the zdd_jl.ipynb notebook to provide an example of using this new function. At present, it doesn't work — the number of compatible pairs doesn't match what we'd expect, and you can play around with checking how many pairs are weight-compatible only, fps-compatible, or cc-compatible, which might be helpful for debugging purposes.

Some additional changes this PR made that are not relevant to Two-Way:

• Added a readable() function for comp_weights
• Changed comp_weights in the recycler to initialize all weights to 0 (rather than undefined)