Interface with xtrack/xfields for tracking with wakes

[lgiacome]

Description

We add all the features needed for using in xtrack/xfields wakes coming from xwakes.

We include a number of tests and examples showing how to track with wakes.
We also add a folder with mpi tests, which will have to be added to the test suite.

Checklist

Mandatory:

• I have added tests to cover my changes
• All the tests are passing, including my new ones
• I described my changes in this PR description

Optional:

• The code I wrote follows good style practices (see PEP 8 and PEP 20).
• I have updated the docs in relation to my changes, if applicable
• I have tested also GPU contexts

[nmounet]

Review finished! many thanks for the great work.

There remains a few minor things to check (many comments are for later PRs)