BNN optimization.

This tackles three issues:

• The aleatoric uncertainty was a single number in the past for the full spectrum. In this version, a number per output is used, allowing the uncertainty to adapt to each case.
• There was a major bug fixed in the hyperprior for the aleatoric uncertainty, which used incorrectly the weight's prior instead of the uncertainty variable. This has now been fixed in the hyperprior calculation.
• The number of neurons have been reduced to simplify the network.