[WIP] Add multi-domain Monge alignment and JCPOT Target shift method

[rflamary]

Adds a multi domain Monge alignement Adapter to SKADA that does the following

1. compte a barycenter of all sources (Gaussian assumtion)
2. Map all data to this barycenter with the clase form
3. At test time it maps all data (form target or source) to the barycenter when calling tranform

Also implement the multisource JCPOT taregt shift method (that should also work for only one source)

Remain to be done

• [x] Proper documentation
• [x] pipeline function wit adapter
• [x] Add references
• [ ] Implement JCPOT