Equilibrium emittance in presence of SR and IBS

[SebastienJoly]

Description

Add a function performing a numerical integration of the emittance evolution equations to find the equilibrium emittance in the presence of SR and IBS.
The equilibrium emittances with IBS and SR are a common metric in the light source community. Until now, it could only be computed with elegant (https://ops.aps.anl.gov/manuals/elegant_latest/elegantsu93.html) and the ZAP code (https://cds.cern.ch/record/185838/files/p289.pdf). It is now possible for Xsuite users to compute it as well.
The current implementation was successfully benchmarked against elegant and yielded the same results.
In addition, two emittance constraints can be selected depending on the use case. Moreover, the numerical integration step is dynamically updated after each iteration, and the integration stops after ensuring it reaches convergence.

I need to create a new folder in xtrack with the BESSY III json file I used as an input in the tests.

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

[fsoubelet]

Working on this with Sebastian, will have a polished version soon enough