Weight sharing and distances in space B

[vdeltatto]

Proposed changes

Added two new options for the DiffImbalance class:

1. possibility of sharing weights among different features in space A (argument 'params_groups')
2. possibility of providing space B in the form of a distance matrix, rather than using features

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)