Merge `bg_nb` to test - Enhance reward balancing feature

[slinlee]

We'll test it in test first. (same as #437)

Enhances auto-norm performance esp for multi-agent simulations.

• fixes freezing + auto-normalization

• ability to disable auto-normalization whilst using reward term interface

• sets policy learning rate to 0.0 until PBT mutation (for establishing initial betas)

• introduces beta learning rate + beta running average (calculated betas become update target)

• beta learning rate schedule

• multi-agent reward term contributions aggregated by maximum contributions among agents

• beta calculation based on 1 / ( max( abs(term_contrib_min), abs(term_contrib_max) ) )

• default beta target set to 1.0