Extensions to relative positional encoding

[cswinter]

Adds several new features to relative positional encoding that allow it to work better in continuous spaces:

• adding a linear projection of the relative position/distance to the relative keys/queries
• partition functions for relative positional encoding based on the relative angle, distance, or polar coordinates
• interpolating between partitions
• (partially implemented) rotating relative positions by the orientation of each entity

Other changes not directly related to relpos encoding:

• improved xprun builds
• new "webpdb" feature to catch crashes and allow for debugging with webpdb
• misc bug fixes