Add a cross-system evaluation #1221

In transfer learning, several articles examine a level of transfer between devices, referred to as cross-system or cross-device:
Melnik2017, Systems, subjects, sessions to what extent do these factors influence EEG data
Wu, 2020, Transfer Learning for EEG-Based Brain-Computer Interfaces A Review of Progress Made Since 2016

The source domains are acquired using devices that do not necessarily share
the same channels and/or sampling rates with one another, or with the target domain.
Typically, these involve different datasets and different subjects.

Inspired by CrossSessionEvaluation and CrossSubjectEvaluation,
the goal of this issue is to add a new class CrossSystemEvaluation to enable the comparison of methods on MOABB.