Add support for Task Arithmetics

[lenglaender]

This PR adds support for various task arithmetic options for LoRA. Until now, our library supported averaging only by linearly combining different adapters. This may be insufficient, especially for LoRA — hence, several publications have proposed other ways to perform task arithmetic.

This PR:

• makes it easier to implement different weighting methods
• adds 2 additional merging methods for LoRA following these papers
• adds method to merge heads
• Docu & notebook