Xaddpy

Fixing incorrect implementation of `sgn_op`. Also, we noticed that SymEngine puts 0 at the LHS for `core.Equality` objects, which we now correctly handle in the updated version.

Perhaps later need to revisit... but Gurobi is optional now.

Fixing the efficiency issue with the prior graph construction code by using a cache to reduce the complexity to linear in size.

Also, `__repr__` now only shows the node ID (for terminal nodes) or the decision ID and the IDs of the low / high branches. Prior implementation hinders debugging sessions when working with large XADD nodes.

Minor updates to lp_util to incorporate the update from PuLP (2.7.0 -> 2.8.0).

Adding the definite integration operation through `XADDLeafDefIntegral` class and its leaf operation implementation.

Plus, introduced the `get_xadd_bounds` function which is used to symbolically compute the lower and upper bounds over a specific variable induced by a given XADD and the variable's inherent numeric bounds.

Fixing minor indentation errors from 0.2.2.